Git diff monaco redesign (#304)

## Summary

Fixes #303.

This PR redesigns the Git Changes Monaco diff gutter so unified and
split view both use a more intentional, space-efficient Monaco
presentation while preserving Monaco's performance on large diffs.

The final behavior includes:

- `Compact` and `Normal` gutter modes for Git Changes
- dynamic gutter sizing based on actual line-number digit counts
- independent original/modified number-column sizing where needed
- split-view fixes for both wasted left inset and line-number/sign
overlap
- persisted gutter-mode selection
- localized user-facing labels for the control

## Visual comparison

### Unified view before

<img width="465" height="353" alt="Unified view before"
src="https://github.com/user-attachments/assets/0c061f25-f20a-4127-a85d-aee1161611c7"
/>

### Unified view after

<img width="634" height="240" alt="Unified view after"
src="https://github.com/user-attachments/assets/f2dfd952-89ed-4fdd-83db-a05f19f023b2"
/>

### Split view before

<img width="596" height="335" alt="Split view before"
src="https://github.com/user-attachments/assets/09bfbe41-9438-4801-b181-49a9d19d5bb8"
/>

### Split view after

<img width="640" height="338" alt="Split view after"
src="https://github.com/user-attachments/assets/fc3618ef-474f-4217-bb21-5ffd53eb4e01"
/>

<!-- If you want to replace these screenshots later, keep the four
sections above and swap the image URLs. -->

## What changed

### Unified view

- added two Git Changes Monaco gutter presentations:
  - `Compact`
  - `Normal`
- kept compact as the tighter single-column-feel unified gutter
- kept normal as the wider Monaco-style unified gutter
- made unified gutter sizing respond to actual line-number digit counts
instead of fixed assumptions
- made normal mode size the visible number columns independently when
one side needs more width than the other

### Split view

- added dynamic split gutter sizing derived from actual before/after
line counts
- made split original and modified number columns size independently
- fixed the modified-pane overlap where larger line numbers could
collide with the `+` lane
- fixed the original-pane wasted left inset caused by Monaco reserving
an empty original-side glyph-margin lane

### Persistence and UI

- persisted the selected gutter mode in preferences so it survives
reloads
- moved the gutter-mode control out of the Git Changes toolbar and into
Appearance settings
- renamed the visible settings options to `Compact` and `Normal`

### i18n

- removed hardcoded user-facing gutter toggle strings
- added localized keys for the gutter control labels and titles used by
the Git Changes surface

## Implementation notes

- Monaco remains the active Git Changes renderer throughout
- gutter sizing logic is centralized in
`packages/ui/src/components/file-viewer/monaco-diff-viewer.tsx`
- CSS is used only for narrow presentation adjustments such as the 4px
left inset and the split original-pane glyph-margin correction
- the persisted gutter-mode preference is the source of truth for the
selected presentation

## Review focus

- unified `Compact` mode should feel tight without clipping or overlap
- unified `Normal` mode should remain wider and readable
- 3-digit and 4-digit line numbers should not collide with the sign lane
- split original pane should no longer show wasted left inset before the
first visible number column
- split modified pane should not leave conspicuous dead space or collide
with the `+` lane as digit counts grow
- selected gutter mode should persist after reload

---------

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

packages/ui/src/components/instance/shell/right-panel/tabs/GitChangesTab.tsx

@@ -384,6 +384,7 @@ const GitChangesTab: Component<GitChangesTabProps> = (props) => {
                 onContextModeChange={props.onContextModeChange}
                 onWordWrapModeChange={props.onWordWrapModeChange}
               />
+
             </>
           }
         list={{ panel: renderGroupedList, overlay: renderGroupedList }}