Cross-platform keyboard shortcut handling

## Summary
On Windows, none of the shortcuts defined with `Cmd` in the command registry fire. For example, Ctrl+A (Select All), Ctrl+B (Clear), Ctrl+X (Cut), Ctrl+Space (Record) and Ctrl+Shift+I (Import Audio) all do nothing. The same commands work from the menu bar. Shortcuts without `Cmd` (e.g. Space, Shift+R) work.

## Environment
- Windows 11 x64
- Built from source at `c51e5d5` (2026-10-08) with `cargo run --release -p soundcraft`

## Steps to reproduce
1. Create an audio track and record or import a clip.
2. Press Ctrl+A, then Ctrl+B.
3. Nothing happens. Edit → Select All, then Edit → Clear, from the menu bar works.

## Cause
`mods_match` in `crates/ui-egui/src/shortcuts.rs` (line 44):

```rust
fn mods_match(want: Modifiers, got: Modifiers) -> bool {
want.command == got.command && want.shift == got.shift && want.alt == got.alt && (want.ctrl == got.ctrl || got.mac_cmd)
}
```

For a `Cmd+…` shortcut, `parse` sets `want.command = true`, `want.ctrl = false`. On Windows and Linux, egui reports the Ctrl key as `command = true` **and** `ctrl = true`, and `mac_cmd` is always false. So `want.ctrl == got.ctrl` is `false == true`, and the match fails for every `Cmd` shortcut.

## Suggested fix
Treat Ctrl as satisfied when the shortcut asks for `Cmd` on non-macOS platforms:

```rust
fn mods_match(want: Modifiers, got: Modifiers) -> bool {
// On Windows/Linux, "Cmd" is the Ctrl key, so Ctrl is expected to be held too.
let ctrl_ok = want.ctrl == got.ctrl || got.mac_cmd ||...