feat(window): multi-window support with a native Window menu

[lampietti]

Description

What

Adds multi-window support: any folder can be opened in its own window, with a native Window menu to list, switch between and arrange those windows.

Why

The app only ever had one window, so reading two sets of docs side by side — or a guide next to its reference — meant closing one folder and reopening the other. Several windows plus a standard Window menu is what a macOS document viewer is expected to offer.

Changes

Multiple windows (99f0331)

• File › New Window (⌘N) opens an empty window; File › Open Folder in New Window… (⇧⌘N) opens a picked folder in its own window, leaving the current one untouched
• New open_new_window command: spawned windows are labelled viewer-N and cascade off the frontmost one
• The pending-open buffer becomes a map keyed by window label, so a window only drains the folder meant for it (it was a single global slot, which two windows starting at once would fight over)
• New PendingOpen::Empty marker keeps a ⌘N window on the welcome screen rather than restoring lastFolder, which would make it a clone of the window it came from
• viewer-* added to the capability's window list — without it, spawned windows come up with no plugin permissions
• resolveInitialView (src/utils.ts) decides what a window shows at startup

Menu commands now target the frontmost window (40756b1)

• app_handle.emit broadcasts to every webview. Harmless with one window; with two, a single ⌘F focused both search fields and one "Open Folder…" raised two pickers
• Every menu command goes through emit_to_focused, and a Finder "Open With" on a running app lands in the frontmost window instead of all of them

Window menu (de45952)

• One checkable entry per open window, checked on the frontmost; selecting an entry brings that window forward
• Rebuilt on every event that can change it: window opened, closed, focused, renamed
• macOS's own set_as_windows_menu_for_nsapp() was tried first and populates nothing here — AppKit only adopts windows created after that call, and never picked up tao's — so the entries are built explicitly
• Windows are titled after the document they show (or their folder), otherwise the list reads as several identical "Markdown Viewer" rows. Renaming goes through a set_window_title command that updates the title and the menu together so they cannot drift

Window arrangement (b5cd5ad)

• Fill, Center, Move & Resize (halves, quarters, Return to Previous Size) and Bring All to Front
• Frames come from NSScreen.visibleFrame rather than Tauri's Monitor::work_area(), which reports the right size but an origin of (0, 0): top-anchored layouts looked fine because macOS clamps them below the menu bar, but bottom halves, bottom quarters and centering all landed one menu-bar-height too high
• Deliberately no accelerators: macOS already binds ⌃⌥+arrows to its own window tiling
• Adds a direct objc2-app-kit dependency (already in the tree transitively via objc2-web-kit)

Docs — README feature bullet, docs/guide/features.md section, keyboard-shortcuts table.

Verification

• npx tsc --noEmit, 311 tests (10 new, covering resolveInitialView and windowTitle), cargo check
• Each of the 4 commits compiles on its own, so the branch stays bisectable
• Arrangement geometry was measured in the running app for every layout — e.g. the bottom half lands at y=1368 on a work area of y=78 h=2580, so its bottom edge is exactly the screen's, and left/right halves meet with no gap
• Per-window folder routing was checked in a running app with three windows (main restoring its folder, one on a given folder, one empty)
• Menu-bar rendering and window switching were verified interactively

Notes for review: lastFolder stays app-wide, so the next launch restores the folder of the last window that navigated. Preferences are read when a window opens, so changing a theme does not retro-apply to windows already open.

Type of Change

• Bug fix
• New feature
• Refactoring
• Documentation

Checklist

• I have tested my changes locally
• I have updated the documentation if needed
• My code follows the existing code style

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