Issue #104: keep file-system calls off the UI thread

Re-applies fix/issue-104-ui-fs on top of the data-safety fixes (integrate/safety):
- availability cache: originals and smart previews are checked on a worker thread; the UI
  answers from the cache (grid, Info panel, photo menu, Missing Photos view)
- import split into ImportJob::prepare (worker) + commit_prepared, with Cancel; Copy keeps
  #96's verified copies
- Find Missing Photos: find_candidates (snapshot, no I/O) + plan_find_missing (all file I/O,
  on a worker, never the session) with #105's matching rules unchanged (name+size+hash;
  renamed by size+hash; single name+size; ambiguous look-alikes skipped and reported; files
  already in use excluded). The plan is applied with library.findMissing {found, missing,
  ambiguous}, re-checked against the catalog then: photos relinked meanwhile are left alone,
  files now in use are not taken. library.findMissing {folder} still runs synchronously.
- Build / Discard Smart Previews with background: true runs on a worker with progress and
  Stop, keeping #106's atomic writes, is_valid checks and `repaired` count (also in
  library.previewProgress); the render worker's smart-preview fallback uses is_valid
- Rename preview planned on a worker (plan_rename_photos) on the same planning core as
  Session::plan_rename_with (MoveFs, case-only respelling by file identity, #95)
- Local folder listing via fs_cached; tests_offline (slow injected probe, frames < 300 ms)

Tests: worker-planned Find Missing equals the synchronous command for a renamed + ambiguous
library; a plan is re-checked when applied; background smart previews repair a damaged proxy.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -30,7 +30,7 @@ mode ([mcp.md](mcp.md)) is a thin layer over this channel. Implementation:
 | `engine.commands` | — | Engine + UI commands: id, label, menu, shortcut, params doc, enabled |
 | `ui.menu.list` | — | Menu entries (flat: id, label, menu path, shortcut, enabled) |
 | `ui.menu.tree` | — | The menu bar as shown (File … Help): items `{id, params?, label, shortcut?, enabled, checked?}`, separators, submenus — the model behind the native macOS menu bar and the in-window menus |
-| `ui.inspect` | — | UI state, window, canvas/image rects, `scroll: {grid, filmstrip}` (scroll offsets in points, `null` until drawn), active photo, selection, perf (`frameMs` = layout, `logicMs` = per-frame logic before it, `updateMs` = both, `maxUpdateMs`, `fps`, render queue …, `gpu` = adapter in use, `gpuReason` = why renders don't use the GPU, `gpuFallback` = latest render redone on the CPU and why — see `docs/gpu-pipeline.md`), status, memory (bytes per cache, see `library.memory`; plus stage caches and textures), `export: {running: {total, done, current} \| null, last}`, `notices` (warnings waiting to be shown, e.g. a damaged settings file; OK = `button:noticeOk`), `quitPrompt` (why quitting was stopped: unsaved changes; `button:quitRetry` / `button:quitAnyway` / `button:quitCancel`) |
+| `ui.inspect` | — | UI state, window, canvas/image rects, `scroll: {grid, filmstrip}` (scroll offsets in points, `null` until drawn), active photo, selection, perf (`frameMs` = layout, `logicMs` = per-frame logic before it, `updateMs` = both, `maxUpdateMs`, `fps`, render queue …, `gpu` = adapter in use, `gpuReason` = why renders don't use the GPU, `gpuFallback` = latest render redone on the CPU and why — see `docs/gpu-pipeline.md`), status, memory (bytes per cache, see `library.memory`; plus stage caches and textures), `export: {running: {total, done, current} \| null, last}`, `notices` (warnings waiting to be shown, e.g. a damaged settings file; OK = `button:noticeOk`), `quitPrompt` (why quitting was stopped: unsaved changes; `button:quitRetry` / `button:quitAnyway` / `button:quitCancel`), `import: {done, total, imported, cancelled} \| null` (an import runs on a worker thread), `tasks` (other background work: `Find Missing Photos`, `Auto Import`) |
 | `ui.widgets` | `{filter?}` | On-screen widgets `{id, rect: [x, y, w, h]}` (screen points) |
 | `ui.clickWidget` / `ui.dragWidget` / `ui.hoverWidget` | `{id, count?, fx?, fy?}` / `{id, toX?, toY?, dx?, dy?, steps?}` / `{id, fx?, fy?}` | Real egui input on a widget (hover: the pointer rests on it, e.g. for preset/profile previews) |
 | `ui.move` / `ui.click` / `ui.drag` | `{x, y, count?, button?}` / `{x, y, toX, toY, steps?}` | Raw pointer input, screen points |