improve delete worktree failure diagnostics (#302)

## Summary
- move delete-worktree failures out of transient toast-only UX and keep
them inline in the delete modal
- add parsed diagnostics for common failure modes, including a short
summary, likely cause, and suggested next step
- make the raw error easier to review and share with raw and sanitized
copy actions

Closes #301.

## BEFORE:

<img width="1127" height="860" alt="image"
src="https://github.com/user-attachments/assets/dd09ba1e-be8c-450c-a1dd-f1cde2a48802"
/>

## AFTER: 

<img width="1384" height="835" alt="image"
src="https://github.com/user-attachments/assets/6b0d1459-21fa-4264-9e54-45540f584538"
/>

## Problem
Before this change, delete-worktree failures were difficult to work
with:

1. The failure message was effectively raw backend or git output.
2. Users had to infer the meaning of the error themselves.
3. The UI did not explain what likely went wrong or what to do next.
4. Sharing the error for debugging was awkward when it included
machine-local absolute paths.
5. The confirmation modal was not being used as the primary diagnostic
surface for a destructive action that frequently fails for
understandable reasons.

This was especially frustrating for common cases such as:
- modified or untracked files in the worktree
- a process still using the worktree directory
- permission errors on Windows
- missing worktree directories or stale worktree records

## What changed

### Modal failure UX
- keep delete failures inline inside
`packages/ui/src/components/worktree-selector.tsx`
- clear modal-local error state when opening or closing the dialog
- keep the success toast on successful deletion, but use the modal
itself for failure presentation

### Human-readable diagnostics
- parse JSON-shaped backend error payloads such as `{"error":"..."}`
before classification
- classify common delete failure patterns into:
  - `localChanges`
  - `inUse`
  - `notFound`
  - `permissionDenied`
  - `unknown`
- render three user-facing lines above the raw error:
  - summary
  - likely cause
  - suggested next step

### Copy flows
- add `Copy error` for the original failure text
- add `Copy sanitized` to redact common absolute path and username
patterns before copying

### Modal content and sizing
- present the target worktree in a simpler two-line summary block
- update the delete description text to plain English: `Deletes this
branch worktree and its local folder.`
- size the delete modal deliberately for desktop use while allowing
vertical expansion to the viewport limit before scrolling

### i18n coverage
- add the new delete diagnostic strings across all currently supported
locales touched by this area:
  - `en`
  - `es`
  - `fr`
  - `he`
  - `ja`
  - `ru`
  - `zh-Hans`

## Why this approach
- It keeps the backend contract unchanged and solves the UX problem
where it occurs.
- It preserves access to the raw failure text instead of hiding
implementation detail entirely.
- It gives users immediate guidance without forcing them to translate
git errors into next actions.
- It improves bug reporting without requiring a separate logging or
export workflow.

## Not included
- server-side preflight guards that block delete when the worktree is
still assigned or in use
- process-aware worktree locking detection
- automatic retry or force-delete-and-retry flows

Those are useful follow-ups, but this PR is intentionally scoped to
failure presentation and debuggability.

## Files changed
- `packages/ui/src/components/worktree-selector.tsx`
- `packages/ui/src/lib/i18n/messages/en/instance.ts`
- `packages/ui/src/lib/i18n/messages/es/instance.ts`
- `packages/ui/src/lib/i18n/messages/fr/instance.ts`
- `packages/ui/src/lib/i18n/messages/he/instance.ts`
- `packages/ui/src/lib/i18n/messages/ja/instance.ts`
- `packages/ui/src/lib/i18n/messages/ru/instance.ts`
- `packages/ui/src/lib/i18n/messages/zh-Hans/instance.ts`

## Validation
- `npm run typecheck --workspace @codenomad/ui`
- `npm run build --workspace @codenomad/ui`
- `npm run typecheck --workspace @neuralnomads/codenomad-electron-app`

## Notes for reviewers
- The error classifier is intentionally heuristic and string-based. It
is meant to improve the common cases without increasing backend
coupling.
- The sanitized copy flow is conservative and focused on path and
username redaction, not full structured log scrubbing.

---------

Co-authored-by: Shantur Rathore <i@shantur.com>

packages/ui/src/lib/i18n/messages/es/instance.ts

@@ -166,6 +166,30 @@ export const instanceMessages = {
   "instanceShell.backgroundProcesses.actions.output": "Salida",
   "instanceShell.backgroundProcesses.actions.stop": "Detener",
   "instanceShell.backgroundProcesses.actions.terminate": "Terminar",
+  "instanceShell.worktree.delete.error.title": "Error al eliminar",
+  "instanceShell.worktree.delete.error.fallback": "Error al eliminar el worktree",
+  "instanceShell.worktree.delete.error.causeLabel": "Causa probable:",
+  "instanceShell.worktree.delete.error.nextStepLabel": "Siguiente paso sugerido:",
+  "instanceShell.worktree.delete.error.summary.localChanges": "Git rechazo la eliminacion de este worktree porque contiene archivos modificados o sin seguimiento.",
+  "instanceShell.worktree.delete.error.summary.inUse": "CodeNomad no pudo eliminar este worktree porque algo sigue usando archivos dentro del directorio.",
+  "instanceShell.worktree.delete.error.summary.notFound": "CodeNomad no pudo eliminar este worktree porque no se encontro el directorio o el registro del worktree.",
+  "instanceShell.worktree.delete.error.summary.permissionDenied": "CodeNomad no pudo eliminar este worktree porque se denego el acceso al directorio.",
+  "instanceShell.worktree.delete.error.summary.unknown": "CodeNomad no pudo eliminar este worktree.",
+  "instanceShell.worktree.delete.error.cause.localChanges": "Cambios locales",
+  "instanceShell.worktree.delete.error.cause.inUse": "Otro proceso esta usando este worktree",
+  "instanceShell.worktree.delete.error.cause.notFound": "Falta el directorio o el registro del worktree",
+  "instanceShell.worktree.delete.error.cause.permissionDenied": "Permisos insuficientes del sistema de archivos",
+  "instanceShell.worktree.delete.error.cause.unknown": "El backend devolvio un error de eliminacion sin clasificar",
+  "instanceShell.worktree.delete.error.nextStep.localChanges": "Activa Forzar eliminacion si quieres descartar los cambios locales, o limpia el worktree e intentalo de nuevo.",
+  "instanceShell.worktree.delete.error.nextStep.inUse": "Cierra terminales, editores, observadores o procesos en segundo plano que usen este worktree y vuelve a intentarlo.",
+  "instanceShell.worktree.delete.error.nextStep.notFound": "Recarga los worktrees y vuelve a intentarlo. Si sigue fallando, inspecciona la ruta del worktree en disco.",
+  "instanceShell.worktree.delete.error.nextStep.permissionDenied": "Revisa los permisos del sistema de archivos y cierra aplicaciones que puedan estar bloqueando este directorio, luego vuelve a intentarlo.",
+  "instanceShell.worktree.delete.error.nextStep.unknown": "Revisa el error sin procesar de abajo para ver los detalles y vuelve a intentarlo despues de corregir el problema indicado.",
+  "instanceShell.worktree.delete.error.copyRaw": "Copiar error",
+  "instanceShell.worktree.delete.error.copySanitized": "Copiar saneado",
+  "instanceShell.worktree.delete.error.copySuccess": "Error de eliminacion copiado",
+  "instanceShell.worktree.delete.error.copySanitizedSuccess": "Error de eliminacion saneado copiado",
+  "instanceShell.worktree.delete.error.copyFailure": "No se pudo copiar el error de eliminacion",
 
   "versionPill.appWithVersion": "App {version}",
   "versionPill.ui": "UI",