COD-121 unified session list: welcome list frontend (slice A, unit 2)

Backs the welcome-screen "Resume Conversation" list with the new
GET /api/sessions/unified endpoint instead of /api/history/sessions, so it
shows the COMPLETE set (live + persisted + non-Claude + closed history)
newest-first with richer context, rather than only Claude transcripts.

- terminal-ui.js: new _fetchUnifiedSessions(); loadHistorySessions() now uses
  it. _buildHistoryItem upgraded to the unified shape (kept backward-compatible
  with the folder-modal's old shape): title = name || firstPrompt || dir; a
  mode badge + a LIVE badge (sources includes 'live'); timestamp from
  lastActivityAt (falls back to lastModified); size only when present; detail
  panel + "View all in this folder" preserved (gated on projectKey). Resume
  branches: an open live session selects its tab, a closed one resumes.
- unified-session-service.ts + endpoint: pass projectKey through the history
  source so the folder drill-down survives.
- styles.css: .history-item-badges / -badge / -badge-live pills.

Verified: tsc 0, lint 0, frontend-syntax + public-asset format clean, service
tests 13/13 (+projectKey), route tests 4/4. Playwright on an isolated beta:
the welcome list renders real items from /api/sessions/unified, and the
renderer produces the tab-name title + codex mode badge + visible LIVE badge,
omits LIVE on closed items, keeps "View all in folder", and routes resume
correctly (open->select tab, closed->resume). Persistent panel + live SSE
status are later units.

src/services/unified-session-service.ts

@@ -29,6 +29,7 @@ export type UnifiedSessionItem = {
   claudeSessionId?: string;
   firstPrompt?: string;
   sizeBytes?: number;
+  projectKey?: string;
   remote?: boolean;
   sources: string[];
   stats?: { memoryMB: number; cpuPercent: number };
@@ -76,6 +77,7 @@ export type HistoryInput = {
   sizeBytes: number;
   lastModified: string;
   firstPrompt?: string;
+  projectKey?: string;
 };
 
 /** Mux process-stat view. */
@@ -147,6 +149,7 @@ export function mergeUnifiedSessions(sources: UnifiedSources): UnifiedSessionIte
     overwrite(item, 'workingDir', h.workingDir);
     overwrite(item, 'sizeBytes', h.sizeBytes);
     overwrite(item, 'firstPrompt', h.firstPrompt);
+    overwrite(item, 'projectKey', h.projectKey);
     const ms = Date.parse(h.lastModified);
     if (!Number.isNaN(ms) && item.lastActivityAt === undefined) item.lastActivityAt = ms;
   }