feat(files): let the path picker jump to a typed path and sort by name or date

The picker's current-folder line was a read-only breadcrumb, so reaching a
deep folder meant tapping through every level, and the listing was fixed to
name order, so the file an agent had just written was somewhere in a
500-entry list.

The current folder is now an editable field: Enter or Go jumps there, a full
file path lands in its folder with that file selected, and a path that does
not resolve keeps the listing you had and says so, instead of the reset to
the root that a stale initialPath gets. A Sort control orders the listing by
name or modified time in either direction, folders always first, and the
choice is remembered per device like the hidden toggle. Each entry shows a
compact modified time (time of day today, month-day this year, else the
date).

GET /api/filesystem/browse stamps every entry with mtimeMs to make that
possible; the stat that already fetched a file's size now serves both, so
it is still one stat per entry. Entries without an mtime (an older server,
the in-container listing) sort after dated ones and then by name.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01McLWqCWBuQYGuPMScb4Aou

src/types/common.ts

@@ -77,6 +77,8 @@ export interface FilesystemBrowseEntry {
   path: string;
   type: 'file' | 'directory';
   size?: number;
+  /** Last-modified time (ms since epoch) of the entry's target; lets the picker sort by date. */
+  mtimeMs?: number;
   symlink?: boolean;
   previewKind?: FilesystemPreviewKind;
 }