feat(linux): harden AppImage release quality

[JNX03]

What changed

• upgrade the packaging CLI from Tauri 2.11.2 to 2.11.4, which fixes absolute .DirIcon and .desktop symlinks in generated AppImages
• add a pinned appdir-lint validation script with checksum verification
• reject missing, broken, or absolute AppImage metadata links before release artifacts are collected
• add a path-scoped Ubuntu 22.04 job that builds the real x64 AppImage, lints it, and confirms a Flowtake window opens under Xvfb
• add regression coverage for the dependency floor and both CI/release gates

Why

The AppImage catalog submission mounted the v1.6.0 artifact but rejected it because .DirIcon was a broken absolute symlink. This is the defect fixed upstream in Tauri #15596. The catalog PR was closed rather than leaving a known-failing submission open.

Local validation

• npm test — 162 passed
• npm run build:frontend
• npm run lint — 0 errors, 41 existing warnings
• bash -n scripts/validate-linux-appimage.sh
• git diff --check

The new Build, lint, and launch AppImage check provides the real Linux artifact validation for this PR.