feat: add full-text search across conversations (#482)

* feat: add full-text search across conversations in sidebar

Add a search mode toggle (Projects/Conversations) to the sidebar search bar.
In Conversations mode, search text content across all JSONL session files
with debounced API calls, highlighted snippets, and click-to-navigate results.

* fix: address PR review feedback - session summary tracking, search sequence invalidation, fallback navigation, SSE streaming

- Track session summaries per-session in a Map instead of file-scoped variable
- Increment searchSeqRef when clearing conversation search to invalidate in-flight requests
- Add fallback session navigation when session not loaded in sidebar paging
- Stream search results via SSE for progressive display with progress indicator

* feat(search): add Codex/Gemini search and scroll-to-message navigation

- Search now includes Codex sessions (JSONL from ~/.codex/sessions/) and
  Gemini sessions (in-memory via sessionManager) in addition to Claude
- Search results include provider info and display a provider badge
- Click handler resolves the correct provider instead of hardcoding claude
- Clicking a search result loads all messages and scrolls to the matched
  message with a highlight flash animation

* fix(search): Codex search path matching and scroll reliability

- Fix Codex search scanning all sessions for every project by checking
  session_meta cwd match BEFORE scanning messages (was inflating match
  count and hitting limit before reaching later projects)
- Fix Codex search missing user messages in response_item entries
  (role=user with input_text content parts)
- Fix scroll-to-message being overridden by initial scrollToBottom
  using searchScrollActiveRef to inhibit competing scroll effects
- Fix snippet matching using contiguous substring instead of
  filtered words (which created non-existent phrases)

* feat(search): add Gemini CLI session support for search and history viewing

Gemini CLI sessions stored in ~/.gemini/tmp/<project>/chats/*.json are now
indexed for conversation search and can be loaded for viewing. Previously
only sessions created through the UI (sessionManager) were searchable.

* fix(search): full-word matching and longer highlight flash

- Search now uses word boundaries (\b) instead of substring matching,
  so "hi" no longer matches "this"
- Highlight flash extended to 4s with thicker outline and subtle
  background tint for better visibility

src/index.css

@@ -904,4 +904,23 @@
   summary svg[class*="transition-transform"] {
     transition: transform 200ms cubic-bezier(0.4, 0, 0.2, 1);
   }
+
+  /* Search result highlight flash */
+  .search-highlight-flash {
+    animation: search-flash 4s ease-out;
+  }
+
+  @keyframes search-flash {
+    0%, 50% {
+      outline: 3px solid hsl(var(--primary));
+      outline-offset: 4px;
+      border-radius: 8px;
+      background-color: hsl(var(--primary) / 0.06);
+    }
+    100% {
+      outline: 3px solid transparent;
+      outline-offset: 4px;
+      background-color: transparent;
+    }
+  }
 }