feat(file-explorer): lazy per-directory watchers + deferred teardown

[tavateva]

Closes #110. Refs #102.

Summary

Replaces the single recursive Watcher(root, {recursive:true, depth:6, limit})
in src/main/event-handlers/filesystem.ts with a Map<path, WatcherEntry> of
non-recursive watchers. fetch-folder-contents(root) closes every open
watcher, resets the aggregate visible-add counter, and starts one watcher on
the root - root is now just a distinguished expanded folder. Two new IPC
handlers drive lazy loading:

• expand-folder(subPath) starts a non-recursive watcher on subPath (or
  cancels a pending teardown if one is queued, making open-close-open
  idempotent).
• collapse-folder(subPath) schedules teardown of the subPath watcher after
  COLLAPSE_TEARDOWN_DELAY_MS (30_000 ms). Cancelled by a subsequent
  expand-folder(subPath). Root is exempt; collapse-of-root is a no-op.

FILE_EXPLORER_WATCH_DEPTH is removed. FILE_EXPLORER_WATCH_LIMIT is kept
but its role changes from a per-watcher watcher-package limit to an
aggregate session budget summed across every open watcher; a watcher's
contribution is reclaimed on teardown.

src/shared/constants.ts is a new dependency-free module holding
COLLAPSE_TEARDOWN_DELAY_MS so both the main process and the renderer import
the same value. tsconfig.node.json and tsconfig.web.json include it;
electron.vite.config.ts gets an @shared alias.

Renderer:

• DirectoryContext exposes expandFolder(subPath) and collapseFolder(subPath).
  Collapse mirrors the main-side timing with a state prune scheduled via
  pruneSubtree(subPath); re-expanding within the window cancels both timers.
  Switching the root clears all pending timers.
• DraggableEntry dispatches expand/collapse alongside its existing visual
  isCollapsed toggle. Idempotent - main-side handlers no-op when state
  already matches. The chevron now renders on every folder (not just
  non-empty ones) so an unopened lazy folder is expandable.
• The nodes: {} IFrame broadcast is unchanged; no plugin-facing API in
  this PR (that lands in Plugin-facing directory request API with recursive semantics (part of #102) #111).

src/preload/index.ts documents the new IPC channels; the electron-toolkit
preload accepts arbitrary channel strings so no per-channel type is required.

Acceptance criteria (from #110)

• Opening a 100k-file folder produces state and events only for root's
  direct entries. (Root is watched non-recursively; batches only include
  root's direct children until subfolders are expanded.)
• Expanding a subfolder streams that folder's direct entries only, no
  recursion. (New non-recursive watcher per expanded folder.)
• Collapsing a subfolder drops its state after the delay unless
  re-expanded first. (Both main-side watcher teardown and renderer-side
  pruneSubtree are scheduled on COLLAPSE_TEARDOWN_DELAY_MS and are
  cancelled by a subsequent expand.)
• File selection, drag-drop between folders, and new-folder uniqueness
  (FileExplorer.tsx:80 isUsed) continue to work. (No changes to the
  NodeChildren shape; handleFSEvent still populates the nested map from
  batch events, only lazily.)
• npm run typecheck passes. (Verified locally; see below.)
• Documented in documentation/docs/index.md (Large Folder Behavior)
  and documentation/docs/reference/technical-constants.md
  (COLLAPSE_TEARDOWN_DELAY_MS added; FILE_EXPLORER_WATCH_DEPTH removed;
  FILE_EXPLORER_WATCH_LIMIT role clarified).

Design notes / minor divergences

• Chevron visibility change. The pre-existing DraggableEntry hid the
  chevron for empty folders (!isEmpty). Under lazy loading a folder starts
  with children: {} until first expand, so !isEmpty would hide the chevron
  on every not-yet-expanded folder and there would be no way to expand it.
  The chevron now shows on every folder regardless. True empty folders are
  still visually distinguished by the emptyFolder style.
• Main-window teardown listener. handleEvents runs before
  createWindow, so getMainWindow() returns undefined at
  handler-registration time. The window closed listener is bound lazily on
  the first fetch-folder-contents call and rebinds if the window is
  recreated (e.g. macOS reactivation).
• Per-watcher addCount also tracks unlinks. When an unlink /
  unlinkDir event fires, both the per-watcher and aggregate counters
  decrement. This keeps the aggregate budget honest across long-lived
  sessions where files come and go without triggering false limit warnings.
• pruneSubtree reducer. Renderer-side prune walks the nested nodes
  map by absolute path equality (not by root-relative path decomposition)
  because FileSystemNode.path is already absolute; this keeps the reducer
  independent of the current root and makes it work uniformly for any
  subPath the main process might collapse.

Blockers / pre-existing issues

• npm run lint fails with ESLint couldn't find an eslint.config.(js|mjs|cjs) file.
  This is the pre-existing ESLint 9 config drift called out in PR chore(constants): named ports w/ env override, tighten iframe allowlist, split memory interval, env-configurable shm_size #109; not
  introduced by this PR and not addressed here.
• Manual smoke via npm run dev was not attempted; the automation host has
  no reachable display. A reviewer can run npm run dev, open a mid-size
  folder, and expand/collapse a subfolder to visually confirm:
  1. Initial open shows only root's direct entries in the panel.
  2. Clicking the chevron on a subfolder loads its direct children.
  3. Collapsing then re-expanding within ~30 seconds is instant with no
     visible re-fetch flicker.
  4. Collapsing and leaving collapsed for >30 seconds, then re-expanding,
     shows a fresh load.

Verification commands

npm ci                # completed
npm run typecheck     # passes (both tsconfig.node.json and tsconfig.web.json)
npm run lint          # fails with pre-existing ESLint 9 config drift; see above

Files changed

• src/main/event-handlers/filesystem.ts - watcher-manager rewrite +
  expand-folder / collapse-folder handlers.
• src/renderer/src/contexts/DirectoryContext.tsx - new
  expandFolder / collapseFolder methods, pruneSubtree reducer,
  timer bookkeeping.
• src/renderer/src/components/MenuPanel/components/FileExplorer/components/DraggableEntry/DraggableEntry.tsx -
  dispatch expand/collapse on chevron click; always show chevron on folders.
• src/shared/constants.ts - new; COLLAPSE_TEARDOWN_DELAY_MS.
• src/preload/index.ts - documenting comment for the new IPC channels.
• tsconfig.node.json, tsconfig.web.json, electron.vite.config.ts -
  include src/shared/**/* and register the @shared alias.
• documentation/docs/index.md - Large Folder Behavior rewritten for the
  lazy model.
• documentation/docs/reference/technical-constants.md -
  FILE_EXPLORER_WATCH_DEPTH removed, FILE_EXPLORER_WATCH_LIMIT role
  clarified, COLLAPSE_TEARDOWN_DELAY_MS documented.