fix: workspace sync gaps and stuck-loading tabs

[anbturki]

Resolves the reported "files don't sync" and "stuck on Loading" feedback. Every bug was reproduced live against a release build before fixing, and every fix was re-verified live afterward (screenshots in the linked issues).

Changes

Workspace sync (useLibrary)

• Rescan the active root on launch, on workspace switch, and on window focus: stale-while-revalidate over the scan cache, so files changed while the app was closed or the workspace was inactive appear without a manual refresh.
• Modify events now schedule a rescan (in-place writes refresh titles, tags, and search); the watcher's other/any rescan sentinels (FSEvents overflow) trigger a rescan instead of being dropped.
• Watches attach with bounded retries and log a warning on failure instead of dying silently.
• fs scope gains explicit $HOME/.docsreader entries: the unix scope glob requires a literal leading dot, so $HOME/** never matched the registry directory and its watch failed on every launch.

Scanner and writes (core)

• validate_phase rejects phase names that collide with the scanner's pruned directories (build, dist, target, venv), reading the same SKIP_DIRS list the scanner uses.
• The scanner follows symlinks (walkdir loop detection verified) and counts walker errors, unreadable files, and oversize files into a new skipped field on ScanResult, surfaced in the explorer footer.

Tabs (useTabs)

• File loads race a 15 s timeout into a friendly, retryable error instead of an eternal spinner; a per-tab load sequence guards timeout/completion ordering; re-activating an errored tab retries the load.

Validation

• 126 frontend tests (13 new), 105 Rust tests (7 new), tsc --noEmit clean, release build clean.
• Live end-to-end verification in an isolated HOME: launch catch-up, live registry sync without focus, symlink visibility, in-place title refresh, and the read-timeout error were all confirmed against the rebuilt binary.

Fixes #18
Fixes #19
Fixes #20
Fixes #21
Fixes #22
Fixes #23