feat: shortcuts dialog, tab context menus, smart status bar, recent files

- ShortcutsDialog: grouped tree of all keyboard shortcuts (Help menu)
- Workspace tab context menu: Rename, Close, Close Others, Close to Right
- SQL query tab context menu: Rename, Duplicate, Close, Close Others
- Smart status bar: right-side connection indicator + selected cell preview
- File → Open SQL File (Ctrl+O), Save (Ctrl+S), Save As, Open Recent
- Recent files persisted to ~/.dbclient/recent_files.json (last 10)
- TableViewer.cell_selected signal wired to status bar cell preview
- EditorTab._filepath tracks the file associated with each query tab

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/config/recent_files.py

@@ -0,0 +1,39 @@
+"""Persistent list of recently opened SQL files (~/.dbclient/recent_files.json)."""
+import json
+from pathlib import Path
+
+_RECENT_FILE = Path.home() / ".dbclient" / "recent_files.json"
+_MAX = 10
+
+
+def load_recent() -> list[str]:
+    """Return up to _MAX recent file paths that still exist on disk."""
+    if not _RECENT_FILE.exists():
+        return []
+    try:
+        with open(_RECENT_FILE, "r", encoding="utf-8") as f:
+            data = json.load(f)
+        return [p for p in data if isinstance(p, str) and Path(p).exists()][:_MAX]
+    except Exception:
+        return []
+
+
+def add_recent(path: str) -> None:
+    """Prepend path to the recent list, deduplicating and capping at _MAX."""
+    recent = load_recent()
+    recent = [p for p in recent if p != path]
+    recent.insert(0, path)
+    _save(recent[:_MAX])
+
+
+def clear_recent() -> None:
+    _save([])
+
+
+def _save(data: list) -> None:
+    _RECENT_FILE.parent.mkdir(parents=True, exist_ok=True)
+    try:
+        with open(_RECENT_FILE, "w", encoding="utf-8") as f:
+            json.dump(data, f, indent=2)
+    except Exception:
+        pass