Stage 8: UI Automation integration (fix smart-copy bug in Notepad / VS Code / Electron)

[zergius-eggstream]

Goal

Replace the clipboard+SendInput approach for reading the user's selected
text with a UI Automation (UIA) path. Keep the old approach as a fallback for
apps that don't expose TextPattern.

Fixes the main user-visible limitation from 0.2.0:

In editors with "smart copy" (Notepad on Windows 11, VS Code, many Electron apps),
Ctrl+C without an explicit selection copies the current line. LightSwitch can't
tell this apart from a real one-line selection, so triggering the conversion hotkey
without an explicit selection in such an editor incorrectly appends a converted
copy of the current line.

UIA's TextPattern.GetSelection() returns the actual selection state, so we
stop confusing "no selection" with "user copied a line."

Acceptance criteria

• perform_conversion (selection-based) uses UIA GetSelection as the primary
  path; if the focused element doesn't implement TextPattern, falls back to
  the existing clipboard+Ctrl+C flow.
• perform_word_conversion uses UIA TextRange::ExpandToEnclosingUnit(Word)
  around the caret as the primary path; falls back to Ctrl+Shift+Left +
  Ctrl+C if UIA isn't available.
• A setting Use UIA when available (checkbox under autostart in the
  settings window) toggles the UIA path on/off. Default: enabled. The
  caption mentions something like "Uncheck if switching behaves oddly in some app."
• Writing text back still uses clipboard+Ctrl+V (consistent, widely supported).
  That's fine because by then we know the real selection from UIA, so the paste
  lands in the right place.
• Clipboard restore still works.
• No "convert all" feature (DocumentRange) — that's a separate future
  improvement, tracked independently.

Out of scope for this issue

• ValuePattern.SetValue for single-line fields — may be added later as an
  optimization for input fields that don't need clipboard.
• "Convert all" via UIA DocumentRange.
• UIA-based language-switch detection (not needed — existing HKL polling works
  for that).

Implementation notes

• New crate feature: Win32_UI_Accessibility in windows.
• New module uia.rs wrapping IUIAutomation, IUIAutomationElement,
  IUIAutomationTextPattern, IUIAutomationTextRange.
• COM initialization: CoInitializeEx(COINIT_APARTMENTTHREADED) on the thread
  that calls UIA (probably the main message loop thread). Must call
  CoUninitialize on shutdown.
• Safe Rust wrappers over COM interfaces — use the windows crate's built-in
  Interface types rather than raw *mut.
• Detailed logging under the "[uia]" prefix so we can diagnose which programs
  expose TextPattern and which don't.

Rollout plan

1. Branch: feat/stage-8-uia.
2. Open draft PR early so CI runs on each push.
3. Manual test matrix: Notepad (old + Win11), WordPad, VS Code, Chrome address
   bar + web textarea, Word, Telegram (Electron), Outlook.
4. Merge to main when manual tests pass and CI is green.
5. Tag v0.3.0 after merge.

[zergius-eggstream]

Shipped in v0.3.0 (merged via PR #2, commit bf0fc5c). All acceptance criteria met; golden tests passed in Notepad/Word/Notepad++/browser.