feat(mobile-web): add long-press message actions — copy, resend, delete

[jarvis24young]

Summary

• Copy text: Long-press any message → "Copy text" copies the full message content to clipboard (strips image markdown references)
• Resend: Long-press a user message → "Resend" re-triggers the query via sendMessage()
• Delete: Long-press any message → "Delete" removes the message from the local Zustand store (soft delete)

Design

• Long-press (500ms, 10px move tolerance) on any message bubble opens a bottom-sheet context menu
• Menu actions are role-sensitive: Resend only appears on user messages
• Toast feedback with automatic timer cleanup on unmount
• Uses same bottom-sheet UX pattern as the session list context menu from feat(mobile-web): add session search, rename, and delete #840

Test plan

• Long-press a user message → menu shows Copy + Resend + Delete
• Long-press an assistant message → menu shows Copy + Delete
• Copy copies clean text (no image refs)
• Resend triggers a new message via sendMessage()
• Delete removes message from the chat view
• Toast feedback appears and auto-dismisses