fix: file-link and session-sidebar review follow-ups from 1.19.0

Five post-merge review items from PRs #306 (clickable file paths) and
#307 (session sidebar):

- constants.js FILE_PREVIEW_EXTENSIONS gains the media extensions it was
  missing vs the single-source sets in attachment-registry.ts (m4v ogv
  ogg oga m4a aac flac opus), so an in-workspace .m4a opens the preview
  player instead of the log viewer; new test/media-extension-parity.test.ts
  pins all three copies (constants.js, panels-ui.js, attachment-registry.ts)
  against each other.
- FILE_PATH_LINK_PATTERN drops `etc` from its root alternation: /etc is
  unconditionally in DEFAULT_BLOCKED_TREES, so every /etc link 403'd.
  Negative cases added to the link-provider and response-viewer tests.
- updateSidebarCount() counts the rows actually on the sidebar list
  (session rows + web-tab rows, minus filtered-out ones) instead of
  this.sessions.size, and applySidebarFilter() refreshes it so the count
  follows the filter box per keystroke.
- The incremental-render connection-line gate now also fires in sidebar
  layout (this._lineageEdgeCount is permanently 0 there), matching the
  strip-scroll listener widened in #307, so a badge changing row heights
  redraws subagent/ultracode connectors.
- isSensitivePath() blocks ~/.claude.json, ~/.claude/settings.json and
  ~/.claude/settings.local.json (credential-bearing by schema), anchored
  to homedir() read at check time so case-level .claude/settings*.json
  files stay servable in the File Viewer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/public/panels-ui.js

@@ -3346,6 +3346,9 @@ Object.assign(CodemanApp.prototype, {
     if (attachmentId) {
       const base = `/api/sessions/${sessionId}/attachments/${encodeURIComponent(attachmentId)}`;
       const IMAGE_EXTS = new Set(['png', 'jpg', 'jpeg', 'gif', 'webp', 'bmp', 'svg']);
+      // VIDEO/AUDIO mirror VIDEO_ATTACHMENT_EXTENSIONS/AUDIO_ATTACHMENT_EXTENSIONS
+      // (src/attachment-registry.ts, the single source); the frontend cannot import
+      // it, so test/media-extension-parity.test.ts pins the copies equal.
       const VIDEO_EXTS = new Set(['mp4', 'webm', 'mov', 'm4v', 'ogv']);
       const AUDIO_EXTS = new Set(['mp3', 'wav', 'ogg', 'oga', 'm4a', 'aac', 'flac', 'opus']);
       // Size when we just registered the file ourselves, so a path opened from a