feat: add opt-in tray startup with a login-only mode

[Adiker]

Summary

ChatGPT Community currently opens its main window on startup. Add the disabled-by-default start-minimized-to-tray feature with two opt-in General settings: start hidden in upstream's existing tray on every cold launch, or only at login autostart. The login-only setting takes priority and leaves manual launches visible; it does not enable autostart itself.

A feature-local launcher hook preserves autostart detection across Chromium's systemd scope transition. Tray/second-instance/deep-link activation reveals the window, and unavailable tray setup falls back to visible startup. Includes localized settings, regression tests, and English/Chinese usage updates.

Scope: the optional app feature and its declarative hook, shared by deb/RPM/pacman/AppImage/Nix builds on amd64/arm64. No new dependencies or changes to shared packaging, updater, or the core patch registry.

Validation

• Real desktop: I tested both modes, including login-autostart-only, successfully on EndeavourOS, KDE Plasma 6.7.5, Wayland, amd64, using the build based on official 26.930.61225.
• node --test linux-features/start-minimized-to-tray/test.js scripts/patch-linux-window-ui.test.js scripts/lib/linux-features.test.js: 62/62 passed.
• bash -n install.sh scripts/lib/*.sh launcher/start.sh.template linux-features/start-minimized-to-tray/launcher-hook.sh; bash tests/scripts_smoke.sh; git diff --check: passed. Changed documentation links, Make commands, and feature ID verified.
• ./install.sh with an isolated feature config containing only start-minimized-to-tray, separate output directories, and CODEX_TARGET_ARCH=amd64 / arm64: both passed against signed stable 26.930.61225; both feature descriptors applied cleanly and the executable hook was staged.
• Broader node --test scripts/patch-linux-window-ui.test.js scripts/lib/linux-features.test.js linux-features/*/test.js: 1003 passed, 18 failed, 2 skipped on Node 26.10.0. All failures reproduced unchanged in the two affected suites on clean upstream cd362d8c (Watchbound staging and the reaper's Wayland-argument expectation).
• Not manually verified: arm64 runtime, other real desktop sessions, or rebuilt deb/RPM/pacman/AppImage/Nix packages. Upstream UI drift rejects an enabled-feature build; tray visibility remains desktop-dependent. Remote CI is pending.

Checklist

• This pull request is ready for review and is no longer a draft.
• I followed CONTRIBUTING.md, kept the change focused, edited source files rather than generated output, and removed unrelated changes.
• If this fixes upstream drift, it targets only the latest signed stable OpenAI Linux package and removes obsolete fallback code and tests. (Not an upstream-drift fix; validated against current signed stable.)
• I added or updated relevant tests, ran the validation listed above, and confirmed that required CI checks pass.
• I reviewed the final diff with my coding agent using maximum reasoning effort, addressed all findings, and reran the relevant tests.

[ilysenko]

Reviewed the opt-in startup/tray integration on the current head. The feature is isolated and disabled by default; signed-stable feature builds, focused regression coverage, reported KDE/Wayland runtime validation, and all required CI checks pass.