Prepare Linux Rev. 2 tray and AppImage fixes

[CryptoKey98]

Fix Linux close-to-tray behavior by preferring a reachable X11/XWayland connection for Manager. Preserve normal backend selection when X11 is unavailable, bound the startup probe, and disable the unsupported close-to-tray option on native Wayland. Builder keeps its normal backend selection.

Give Manager and Builder separate Linux app IDs matching their desktop launchers and X11 window classes. Add matching identities and icons to generated portable shortcuts.

Identify AppImage installations from their headers, including renamed legacy executables. Show AppImage labels in installation selectors and package details while preserving selected copies and portable letters. Add regression coverage for migration, reopening, ordinary portable replacements, and backend selection. Update the Linux documentation and changelog. Add a Linux-only 0.8.3 Rev. 2 patch-notes section at the bottom of What's New. Track dismissal by version and revision so revision updates reopen it once; legacy version-only records count as Rev. 1, and new baseline records remain readable by older managers.

Validation:

• Ubuntu: 230 automated tests passed; strict Clippy passed.
• Ubuntu desktop: closing hides the XWayland window, tray Activate reopens it, and tray Exit quits.
• Native Wayland startup with an invalid DISPLAY passed.
• Manager and Builder runtime identities verified on XWayland (WM_CLASS) and native Wayland (set_app_id). Launcher regression test failed before the fix and passed afterward.
• Windows: 206 library tests passed, one installer test ignored; all-target strict Clippy passed.
• Rust formatting, release validation tests, and release metadata validation passed.
• What's New dismissal regression tests: 4 passed on Ubuntu Rev. 2 and 4 on Windows Rev. 1. Legacy and revision cases failed before the fix and pass afterward; focused Clippy passed on both platforms.
• Focused independent code review completed.

Limitations:

• Fedora and Arch desktop behavior was not retested in this pass; macOS runtime was not tested.
• A deliberately nonresponsive fake X11 server can still stall graphics startup, including Builder's unchanged backend path. This is separate from the normal desktop scenarios verified above.

Release scope:

• Prepare Linux-only 0.8.3 Rev. 2; do not publish it yet.
• No version bump or change to the published v0.8.3 tag.
• AGENTS.md and the saved local patch are excluded.