feat: switch draw mode — overlay hide/show toggle

[ajianaz]

Changes

Core: Switch between draw overlay and normal app usage

• Draw mode ON → overlay visible, captures mouse, user can scribble
• Draw mode OFF → overlay hidden, user interacts with apps behind it
• Strokes preserved when toggling — they reappear when draw mode is re-enabled

Three ways to toggle

1. Global shortcut Alt+Shift+D — works from anywhere, even when using another app
2. Tray menu → Toggle Draw Mode
3. FAB button — red ✕ circle at bottom-left when in draw mode

Technical changes

• Rust: hide_overlay() function (set click-through → hide window)
• Rust: show_overlay() function (show → focus → always-on-top → capture mouse)
• Rust: Global shortcut re-enabled with proper error handling
• Rust: Startup uses Accessory policy (hidden from dock until draw mode)
• Config: visible: false, skipTaskbar: true in tauri.conf.json
• Capabilities: Added set-always-on-top, allow-show, allow-hide
• Frontend: Removed debug overlay, cleaned up unused variables
• Frontend: FAB stop button (red circle) replaces text button

Testing

• Frontend check: 0 errors, 0 warnings
• Rust compile (needs macOS/Linux GTK deps — will run in CI)
• Manual: toggle via shortcut from another app
• Manual: strokes persist after hide/show

[ajianaz]

Closing for now — will revisit. UX gap: no way to enter draw mode on develop base (removed Start Draw button without this PR's shortcut toggle).