Add shared boards, clean paginated navigation, and system desktop appearance

[rafiki270]

Summary

• Shared task boards: persistent board IDs, agent creation/posting tools, scoped board history/search, and a transactional migration of existing messages into General.
• Navigation: newest-first board rows with chevrons, click-through messages, Back navigation, search on both levels, and bounded First/Previous/Next pages for boards, messages, audit history, sessions, and network services.
• Desktop appearance: supplied icon throughout macOS, Windows, and Linux packages and the dashboard; system light/dark colors synchronized with native window chrome; full-height sidebar with independent content scrolling.
• Content: remove workspace banners, thread shortcuts, decorative actions, metrics, and slogans; keep technical message metadata in the Details disclosure.
• Documentation: update the brief, API, storage/migration guidance, verification record, and copy-paste implementation brief.

Testing

• Local lint and 16 backend/MCP tests passed, including v1 migration, cross-board authorization, replay safety, literal board search, bidirectional pagination through 27 pages, and two real stdio MCP processes sharing a task board.
• All four source and packaged Electron flows passed on macOS, including light/dark transitions, newest-first ordering, search, Back, bounded pages, and narrow-window sidebar geometry.
• Mac ARM64 ZIP built; the rebuilt application was launched and checked through native accessibility and screenshots. Normal quit/relaunch and the backed-up local database migration were verified.
• Windows native lint, all 16 backend/MCP tests, all four source GUI flows, NSIS packaging, packaged MCP, and all four packaged GUI flows passed.
• Three-platform CI passed with explicitly sandboxed source and packaged GUI flows. Native Ubuntu lint/backend tests, AppImage and DEB builds passed. Native Ubuntu GUI launch still requires administrator configuration of the Chromium sandbox helper; Linux CI configures the helper and explicitly enables sandboxing.
• Installer installation/upgrade, signing/notarization, store/Homebrew publication, and macOS Intel builds remain unverified.