Add rebindable key and controller bindings with settings page

[vanlueckn]

Summary

• Introduced crates/app/src/bindings.rs: a complete rebinding system for ~60 controls grouped into buttons (press/release) and levers (position-based)
• Settings page under Settings → Key bindings with per-control rows showing keyboard and gamepad inputs
• Enter to rebind (captures next key or controller button), Backspace to clear, Esc to cancel
• Bindings stored in settings file under [controls] section; only non-default rows are persisted
• Lever controls (throttle, brake valve, direct brake) bind to analog axes/triggers instead of keys; unbound by default to prevent accidental control hold
• Look (right stick) and walk (left stick) are always-on, never bindable
• All controllers answer to the same bindings; one key can only work one control
• Updated menu and UI systems to support controls page navigation
• Localization: added i18n keys for all ~60 controls and UI labels in both en/main.ftl and de/main.ftl
• Key help (F5) dynamically rewritten when bindings change
• Cleaned up audio module (removed unnecessary shots cache)

Testing

• Verify controls page renders correctly with keyboard + gamepad inputs side-by-side
• Test rebinding a button control: Enter, press a key, confirm it takes effect
• Test rebinding a lever control: Enter, move analog stick/pull trigger, confirm axis is bound
• Test clearing a binding: Enter, press Backspace, confirm input is removed
• Test canceling a rebind: Enter, press Esc, confirm original binding unchanged
• Test one-key exclusivity: bind a key to one control, rebind it elsewhere, confirm it's removed from the first
• Verify settings file [controls] section only contains changed rows
• Launch with fresh settings and verify default bindings work
• Test from both pause menu and Settings screen
• Verify key help (F5) reflects current bindings after changes
• (Multiplayer) Confirm bindings are local only; CabInputs positions still network correctly