feat: route press_key chords through the portal keyboard session

[avifenesh]

Closes #50 (thanks @maxburto for the detailed report).

What

press_key previously parsed the chord and unconditionally invoked ydotool, even when an XDG RemoteDesktop keyboard session was already active — the same session type_text already obtains and uses. On Plasma Wayland this meant installing ydotoold + /dev/uinput access solely for key combos.

How

• New key_chord helper parses a chord (Ctrl+Shift+P) into evdev modifier codes + key code; key_sequence (the ydotool encoding) is now a thin wrapper over it, so both backends share one parser and the key grammar stays identical.
• press_key sends the chord through the existing press_keycode_chord portal call when the new chord policy prefers the portal, with the result message identifying the backend ("Action sent through the remote desktop portal.").
• Chord policy (should_prefer_portal_keyboard_for_chords): on any Wayland session — including Plasma, unlike the literal-text policy which routes KDE to the clipboard backend — the portal is used when a keyboard session is already cached (reusing the consent the user granted, even if ydotool is present), when ydotool is absent, or when COMPUTER_USE_LINUX_FORCE_PORTAL_KEYBOARD is set. COMPUTER_USE_LINUX_FORCE_YDOTOOL_KEYBOARD still pins ydotool.
• Portal send failures clear the cached session and surface the error explicitly (no silent fallback after a failed portal send, matching type_text semantics). If no session can be established, ydotool remains the fallback.
• Focus verification before input is unchanged, and focus/input-landing notes attach to the portal path too.

Acceptance mapping (from #50)

• Plasma Wayland + keyboard-enabled session + no ydotool → press_key Ctrl+S goes through the portal. ✓ (policy prefers portal when ydotool socket is absent, and Plasma is no longer excluded for chords)
• Consent absent / portal send fails → explicit error, cached session cleared. ✓
• Focus verification preserved. ✓

Validation

• cargo test --lib: 141 passed (adds key_chord_splits_modifiers_and_key).
• cargo fmt --check, cargo clippy --all-targets -- -D warnings: clean.
• Not live-tested on Plasma hardware — @maxburto, if you can try this branch on your setup before it ships, that would be a great confirmation of the acceptance case.

[gemini-code-assist]

Code Review

This pull request introduces support for sending key chords through an active remote desktop portal keyboard session on Wayland, falling back to ydotool when unavailable. It refactors key parsing by introducing a key_chord helper to split modifiers and keycodes, updates key_sequence to leverage this helper, and adds corresponding unit tests. There are no review comments, so I have no feedback to provide.

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