Feature/0.1.2: with improved post reading and Linux packaging

[Shin-Aska]

Summary

• Add Settings with a configurable post cache limit, bound retained feed and notification posts, and improve responsive layouts.
• Improve post reading with an image viewer, content warning disclosures, Bluesky HLS video playback, thumbnails, and live social action updates.
• Harden Bluesky session and video authorization. Fix Mastodon reply handling, pagination cursors, and repost conflicts.
• Update AppImage packaging and add startup smoke checks for Ubuntu 22.04 and 24.04. Add v0.1.2 release notes and backend documentation.

Verification

• Passed: npm run check, npm run build, Rust formatting, and Clippy.
• Passed: 29 Bluesky tests and 2 targeted UI tests.
• Full Rust suite: 103 passed, 2 failed.

Open checks

• mastodon_status_text_preserves_html_paragraphs_and_entities still fails: HTML entities and paragraph breaks are not preserved.
• draft_and_media_survive_database_reopen fails on Windows because cleanup encounters a file in use.
• AppImage smoke jobs were added to CI but were not run locally on Windows.