refactor(web): harden direct message dialog

[RomeoCavazza]

What changed

• migrates Direct Message from a manual overlay to the shared Radix Dialog shell
• makes each roster Message button the real trigger for focus restoration
• loads a conversation only while its dialog is open
• keeps the thread as the only scrollable region in a bounded dvh layout
• announces only unseen messages from the currently open peer after a silent baseline
• coalesces message batches and deduplicates repeated query invalidations
• announces send and load failures without exposing WebSocket transport state

Product boundaries

• no API, PostgreSQL, RBAC or WebSocket protocol change
• no global chat abstraction
• existing one-to-one operational messaging remains intact

Validation

• 86 Vitest tests
• 40 Playwright flows
• real Observer send and Responder to Manager HTTP plus WebSocket receive
• Escape, focus restoration, pending close, error announcement and 320 px geometry
• Next.js build, ESLint, Prettier, TypeScript, Stylelint and Knip
• Rust fmt, Clippy, audit and deny
• deterministic reset to 6 stable private messages