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/sensitive-path.ts

@@ -29,6 +29,9 @@
  * symlink pointing at a sensitive target is also caught.
  */
 
+import { homedir } from 'node:os';
+import { join } from 'node:path';
+
 const SENSITIVE_PATTERNS: RegExp[] = [
   // System account databases.
   /^\/etc\/shadow$/,
@@ -99,10 +102,26 @@ const SENSITIVE_PATTERNS: RegExp[] = [
   /\/\.codeman[^/]*\/intents\.json$/,
 ];
 
+/**
+ * Claude config members that are credential-bearing ONLY under the user's real
+ * home directory: `~/.claude/settings.json` can hold `env.ANTHROPIC_API_KEY`
+ * and `apiKeyHelper` by schema (settings.local.json shares that schema), and
+ * `~/.claude.json` holds account/OAuth-adjacent state. A blanket
+ * `/\.claude\/settings\.json$/` would also block every CASE-level
+ * `.claude/settings.json`, which users legitimately view and edit in the File
+ * Viewer (model override, hooks) — so these are anchored to homedir(), read at
+ * CHECK time inside isSensitivePath, never captured at module load (wrong for
+ * anything that changes HOME later, e.g. per-file test fixtures — same
+ * reasoning as the `.ssh/` note above).
+ */
+const HOME_SENSITIVE_MEMBERS = ['.claude.json', '.claude/settings.json', '.claude/settings.local.json'];
+
 /**
  * Returns true if the given ABSOLUTE, symlink-resolved path matches the
  * sensitive-file blocklist and must not be served to the browser.
  */
 export function isSensitivePath(absPath: string): boolean {
-  return SENSITIVE_PATTERNS.some((pattern) => pattern.test(absPath));
+  if (SENSITIVE_PATTERNS.some((pattern) => pattern.test(absPath))) return true;
+  const home = homedir();
+  return HOME_SENSITIVE_MEMBERS.some((member) => absPath === join(home, member));
 }