feat: add View all in folder modal for Resume Conversation (#94)

Drill into a single project's complete history when the homepage's
3-per-project dedup hides older conversations.

- Backend: /api/history/sessions accepts projectKey/offset/limit;
  single-folder mode bypasses the 50-cap and returns { sessions, total }.
  projectKey is validated against ^[A-Za-z0-9_-]+$ to prevent traversal.
- Frontend: detail panel adds "View all in this folder" button that
  opens a modal listing 20 sessions per page with Show more pagination.
- Modal items reuse _buildHistoryItem with showViewAll:false to avoid
  recursive entry points.

Co-authored-by: Teigen <teigen@TeigendeMac-mini.local>

src/web/public/styles.css

@@ -2369,6 +2369,56 @@ body {
   color: var(--text);
 }
 
+.history-detail-actions {
+  margin-top: 0.5rem;
+}
+
+.history-view-all-btn {
+  width: 100%;
+  padding: 0.45rem 0.75rem;
+  background: rgba(99, 179, 237, 0.08);
+  border: 1px solid rgba(99, 179, 237, 0.25);
+  border-radius: 6px;
+  color: rgba(99, 179, 237, 0.95);
+  font-size: 0.78rem;
+  font-weight: 500;
+  cursor: pointer;
+  transition: background var(--transition-smooth), border-color var(--transition-smooth);
+}
+
+.history-view-all-btn:hover {
+  background: rgba(99, 179, 237, 0.15);
+  border-color: rgba(99, 179, 237, 0.5);
+}
+
+/* Folder history modal */
+.folder-history-modal .modal-body {
+  padding: 0.75rem 1rem 1rem;
+}
+
+.folder-history-subtitle {
+  color: var(--text-muted);
+  font-size: 0.78rem;
+  font-family: 'SF Mono', Menlo, Consolas, monospace;
+  word-break: break-all;
+  margin-bottom: 0.75rem;
+  padding-bottom: 0.5rem;
+  border-bottom: 1px solid rgba(255, 255, 255, 0.06);
+}
+
+.folder-history-list {
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+}
+
+.folder-history-empty {
+  padding: 2rem 0.5rem;
+  text-align: center;
+  color: var(--text-muted);
+  font-size: 0.85rem;
+}
+
 .welcome-hint {
   color: var(--text-muted);
   font-size: 0.8rem;