chore(ui): finalize timeline selection audit fixes

Complete re-review of PR #188 (commits 224cab6 feature + 2c27fc5 perf/i18n follow-up). Gatekeeper focus: standards, correctness, perf/complexity, and translation completeness.

What this changes (pre -> post)

Pre: timeline primarily navigation/hover preview; bulk delete selection message-level and token metrics tied to backend assistant output tokens (missing tool payload weight).

Post: segment-level timeline selection + range (Shift) + toggle (Ctrl/Meta) + mobile long-press; histogram ribs overlay showing relative + absolute (~10k cap) token weight; assistant-turn grouping to avoid adjacency bugs; bulk-delete toolbar shows Before / Selection / After token pills.

Code standards / correctness

OK: Solid signal/memo/effect patterns with cleanup; no obvious lifecycle leaks. Grouping avoids adjacency overlap by mapping messageId to turns.

Fix: selection-id stability is mitigated by pruning stale ids after segment rebuilds; long term stable ids from part ids/toolPartIds remain recommended.

Fix: token counts now share getPartCharCount in both x-ray overlay and bulk-delete toolbar, keeping estimates consistent with live store updates.

Performance / complexity

OK: O(n^2) hotspots removed for liveSegmentChars and selectedTokenTotal. groupRole + deleteUpTo hover checks now memoize messageId sets/maps.

Note: getPartCharCount can be heavy for large tool payloads but remains gated behind selection mode.

CSS / UI integration

Fix: x-ray token label now uses theme tokens instead of hard-coded colors. Delete toolbar now uses menu-based controls with selection-mode toggle.

i18n

Fix: selection hint now renders Cmd/Ctrl via localized modifier placeholder; all locales updated.

packages/ui/src/lib/i18n/messages/en/messaging.ts

@@ -86,10 +86,14 @@ export const messagingMessages = {
   "messageSection.bulkDelete.toolbarAriaLabel": "Selected messages ({count})",
   "messageSection.bulkDelete.deleteSelectedTitle": "Delete selected messages",
   "messageSection.bulkDelete.selectAllTitle": "Select all messages",
+  "messageSection.bulkDelete.moreOptionsTitle": "More options",
+  "messageSection.bulkDelete.selectionModeLabel": "Selection",
+  "messageSection.bulkDelete.selectionModeAll": "All",
+  "messageSection.bulkDelete.selectionModeTools": "Tools only",
+  "messageSection.bulkDelete.selectionHint": "{modifier}+Click toggle · Shift+Click range · Esc clear",
   "messageSection.bulkDelete.cancelTitle": "Cancel selection",
   "messageSection.bulkDelete.failedTitle": "Delete failed",
   "messageSection.bulkDelete.failedMessage": "Failed to delete selected messages",
-  "messageSection.bulkDelete.selectionHint": "Ctrl+Click toggle \u00b7 Shift+Click range \u00b7 Esc clear",
   "messageItem.status.queued": "QUEUED",
   "messageItem.status.generating": "Generating...",
   "messageItem.status.sending": "Sending...",