Fix Linux AppImage startup and complete 0.1.2 cache limits

[Shin-Aska]

Motivation

• Prevent AppImage startup freezes by eliminating incompatible mixes of bundled GLib/GIO/nghttp2 with host libraries and by supplying a coherent media runtime for WebKit/GStreamer.
• Make the post-response memory-cache robust and configurable while bounding mounted-view retained post state to a deterministic limit.
• Add automated AppImage extraction audits and startup smoke tests so future regressions in loader/ABI/plugin interaction are detected in CI.

Description

• Enforce a consistent AppImage runtime strategy by excluding host ABI boundary libraries via a tauri build wrapper and vendoring vetted linuxdeploy plugin scripts; add scripts/tauri.mjs and scripts/prepare-appimage.mjs to pin and patch gtk/gstreamer bundling behavior and avoid bundling GLib/GObject/GIO/nghttp2.
• Enable Tauri AppImage media-framework bundling and require the runtime GStreamer plugins while adding scripts/audit-appimage.sh and scripts/smoke-appimage.sh to audit extracted libs and run a headless startup smoke test under xvfb + dbus-run-session.
• Harden packaging CI by installing missing helper tools (file) and adding an appimage-smoke job that builds the AppImage and runs the audit and smoke scripts on Ubuntu 22.04 and 24.04.
• Finish and wire a deterministic mounted-view retention policy with src/services/social/retention.ts (500 post-bearing rows per view) and apply it in feed and notification presentation code, while keeping the existing 96-entry SocialReadCache and adding the configurable response-post budgets plumbing.
• Make preference persistence tolerant: readPostCacheLimit and savePostCacheLimit now gracefully fall back if localStorage is invalid or unavailable, and the UI/tests were updated to reflect the 0.1.2 version and the new Settings entry.
• Add and extend Playwright tests (tests/social-cache.spec.ts, tests/workspace.spec.ts) to cover LRU eviction by aggregate post budget, immediate eviction when lowering limits, Off semantics, oversized response handling, and deterministic mounted-state bounds; add docs/releases/v0.1.2.md and document the runtime strategy in docs/architecture.md.

Testing

• Frontend: ran npm ci, npm run lint, npm run check, npm run build and npm run test:ui with the full Playwright suite, producing 87 passing UI tests.
• Desktop/Rust: ran cargo +1.98.1 fmt --check, cargo +1.98.1 clippy and cargo +1.98.1 test (with NO_PROXY=localhost,127.0.0.1,::1 to avoid an environment proxy routing loopback); unit tests passed (91 Rust tests passed).
• AppImage packaging: ran npm run tauri -- build --ci --bundles appimage -- --locked to produce src-tauri/target/release/bundle/appimage/Threadline_0.1.2_amd64.AppImage (SHA-256 bf028dac97ffd45052185ebb7c5a3c8c1eae9ec43e5870a56a8fb813fb77e121).
• AppImage verification: scripts/audit-appimage.sh (extracted library audit) and scripts/smoke-appimage.sh (headless startup smoke) were run against the produced AppImage and succeeded, confirming no undefined symbol, mismatched GIO modules, missing autoaudiosink, or GLib-GObject-CRITICAL diagnostics were emitted.

---

Codex Task