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/styles/messaging/message-selection.css

@@ -111,6 +111,90 @@
   color: var(--text-primary);
 }
 
+.message-delete-mode-button--menu {
+  color: var(--text-muted);
+}
+
+.message-delete-mode-button--menu:hover {
+  color: var(--text-primary);
+}
+
+.message-delete-mode-menu-container {
+  position: relative;
+}
+
+.message-delete-mode-menu {
+  right: 0;
+  bottom: calc(100% + 6px);
+  min-width: 150px;
+  width: max-content;
+  max-width: min(70vw, 220px);
+  padding: 2px 0;
+}
+
+.message-delete-mode-menu-divider {
+  height: 1px;
+  margin: 3px 0;
+  background-color: var(--border-base);
+}
+
+.message-delete-mode-menu-row {
+  display: flex;
+  flex-direction: column;
+  gap: 3px;
+  padding: 2px 8px 6px;
+}
+
+.message-delete-mode-menu-label {
+  font-size: 12px;
+  color: var(--text-muted);
+  text-align: center;
+}
+
+.message-delete-mode-menu-toggle {
+  display: inline-flex;
+  border-radius: 999px;
+  border: 1px solid var(--border-base);
+  background-color: var(--surface-secondary);
+  overflow: hidden;
+  width: 100%;
+}
+
+.message-delete-mode-menu-toggle-button {
+  padding: 2px 8px;
+  font-size: 12px;
+  color: var(--text-muted);
+  background: transparent;
+  border: none;
+  cursor: pointer;
+  transition: color 0.15s ease, background-color 0.15s ease;
+}
+
+.message-delete-mode-menu-toggle-button[data-mode="all"] {
+  flex: 0 0 auto;
+  min-width: 56px;
+}
+
+.message-delete-mode-menu-toggle-button[data-mode="tools"] {
+  flex: 1 1 auto;
+}
+
+.message-delete-mode-menu-toggle-button[data-active="true"] {
+  color: var(--text-primary);
+  background-color: color-mix(in oklab, var(--accent-primary) 18%, transparent);
+}
+
+.message-delete-mode-menu .dropdown-item {
+  width: calc(100% - 8px);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  margin: 0 4px;
+  padding: 3px 8px;
+  font-size: 12px;
+  border-radius: 6px;
+}
+
 .message-delete-mode-button:focus-visible {
   outline: none;
   box-shadow: 0 0 0 2px color-mix(in oklab, var(--accent-primary) 45%, transparent);

packages/ui/src/styles/messaging/message-timeline.css

@@ -357,10 +357,10 @@
   font-weight: 600;
   font-variant-numeric: tabular-nums;
   line-height: 1;
-  color: #1a1a2e;
-  background: #ffffff;
+  color: var(--text-primary);
+  background: var(--surface-base);
   padding: 1px 5px;
-  border: 1px solid #1a1a2e;
+  border: 1px solid var(--border-base);
   border-radius: 999px;
   white-space: nowrap;
   pointer-events: none;