installer: add opt-in Chrome launcher setup and recovery

[AoTofu]

Native Chrome/Chromium users currently have to maintain desktop overrides for the documented VA-API settings themselves. If an override calls a custom browser wrapper that stops working after a Chrome update, there is also no installer command to return that entry to the packaged browser while retaining VA-API configuration.

This adds two explicit, launcher-only commands:

./install.sh --configure-chrome-launcher google-chrome.desktop
./install.sh --restore-chrome-launcher google-chrome.desktop

• Configuration creates a user override from the matching system desktop template and refuses to overwrite an existing entry. Restoration backs up the previous contents to a unique file before replacing them.
• Applies the existing README's decode flags, LIBVA_DRIVER_NAME=nvidia, the driver directory, and NVD_BACKEND=direct to the main command and desktop actions. NVD_DRIVER_DIR can select a local build.
• Preserves localized template names, incognito actions and desktop field codes; handles both desktop-entry escaping layers. Disables D-Bus activation in the generated entry so its Exec environment takes effect.
• Supports directly launched native browsers. Missing/unsupported templates and backup failures leave the current entry unchanged; Flatpak, Snap and wrapper templates are rejected.
• Ordinary installation behavior is unchanged. These optional commands use Python 3's standard library and exit before build/install operations.

The branch is based directly on upstream master (982ba1c). It changes five installer/documentation/test/CI files, with no driver-source changes or dependency on fork-specific export-layout settings.

Validation:

• bash -n install.sh and git diff --check passed.
• All 11 Python tests passed locally, including backup/write failure cases, isolated XDG directories, command-mode isolation, desktop-file-validate, and actual GIO argument delivery with spaces/quotes/backslashes/dollar signs/percent signs in paths. The suite also passed with display/session-bus environment variables removed. GIO and desktop-file-utils checks skip explicitly when those tools are unavailable.
• Added a separate Ubuntu CI job for the launcher tests.
• Built upstream with Meson/GCC and launched Chrome 153.0.8010.36 through the generated command, using an isolated profile and that upstream library. On Fedora 44 / Wayland / RTX 5080 / NVIDIA 610.57.04, CDP reported VaapiVideoDecoder, process mappings confirmed the selected library, and a three-second VP9 1080p playback produced 90 frames with zero drops.
• The smoke harness also reproduced an existing seek-frame mismatch (6/10 seeks); this PR addresses launcher configuration, and does not claim to fix decoder/image correctness.