Make the interactive Browser honor configured keyboard layouts

[Jehu]

The interactive Browser window always ran with a US XKB layout, so non-US keyboards could not type their printed characters: German Mac users could not type @ with Option+L, and umlauts, brackets, and AltGr symbols landed on wrong keys or vanished. The Browser now applies a configured XKB layout to its private display, so typed characters match the physical keyboard.

What changed

• New Browser settings keyboard_layout / keyboard_variant (e.g. de + mac). Values are normalized to safe XKB tokens; empty keeps the current US behavior. Layout changes flow through browser_runtime_config, so saving a new layout restarts internal runtimes, matching the existing proxy-settings semantics.
• Browser startup applies the layout to the private Xvfb display with setxkbmap and pins it on the Xpra shadow server (--keyboard-sync=no, --keyboard-layout, --keyboard-variant), so connecting HTML5 clients cannot override the server layout.
• The fallback canvas input path no longer discards Alt/AltGr combinations that produce printable characters; the previous code returned early on any altKey event.
• The settings UI offers free-text layout and variant fields with a link to the official xkeyboard-config list.

Tests

• pytest tests/test_browser_agent_regressions.py -q — 129 passed (config defaults, token normalization, restart semantics, Xpra arg generation, setxkbmap command generation, unchanged extension-path contract).
• Manually verified on a live Dockerized runtime: with de/mac configured, Option+L types @ in the interactive Browser window; xmodmap on the private display confirms Level3 of L maps to at.

[3clyp50]

Hi @Jehu, thank you for the contribution. I rebased the PR onto the current ready branch and added a small follow-up commit to tighten fallback canvas keyboard handling. AltGraph and macOS Option combinations that produce text are still forwarded, while ordinary Windows and Linux Alt shortcuts are no longer inserted as literal characters. I also added focused regression coverage. Merging now.