Choose Windows-safe default hotkeys

[martinbha]

Goal

Ship Windows default hotkeys that do not collide with AltGr or platform conventions.

Context

The shared defaults in crates/panes-platform/src/lib.rs use Control+Alt+<key> for every binding. On Windows, Ctrl+Alt is equivalent to AltGr, which international keyboard layouts use to type characters (AltGr+4 etc.), so registering Control+Alt+Digit1..6 and letter combinations shadows normal typing for those users. Windows also has its own conventions (Win+arrows for snapping) and reserved combinations (Ctrl+Alt+Delete). The config system from #5 lets users rebind, but defaults are what most people keep.

Scope

• Decide per-platform default bindings: keep the current set on macOS, choose a Windows-safe set (avoid Ctrl+Alt with digits/letters; consider Win- or Ctrl+Win-based chords, noting which Win combinations the OS reserves).
• Restructure default_hotkey_bindings() to return platform-appropriate defaults (compile-time cfg or a platform parameter).
• Keep config override behavior unchanged; document the per-platform defaults in docs/config.example.toml and the README.
• Update the binding-consistency tests to cover both default sets.

Implementation Notes

• Sequence with Add Windows tray menu and hotkey runtime #7 (Windows tray/hotkey runtime); the decision is needed before Windows defaults ship, not before.
• global-hotkey supports the Super/Meta modifier for Win-key chords; verify which Win+key combinations Windows Explorer already claims (Win+arrows are taken by Snap; Ctrl+Win+arrows by virtual desktops).

Acceptance Criteria

• Windows defaults contain no Ctrl+Alt+digit/letter combinations.
• macOS defaults are unchanged.
• Tests assert each platform's default set parses and has no duplicates.
• Docs state the defaults per platform.

Validation

• cargo fmt --all -- --check
• cargo check --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• Manual Windows check with an AltGr layout (e.g. German or Polish) once the Windows runtime exists.