fix: accept WebSocket before validation to prevent opaque 403 errors

[nioasoft]

Summary

• All 5 WebSocket endpoints were closing connections before accept(), causing Starlette to return opaque HTTP 403 errors with no useful information to the client
• Moved websocket.accept() before all validation checks so errors are sent as JSON messages before clean close with 4xxx codes
• Client-side hooks now skip reconnection on application-level error codes (4000-4999) to prevent futile retry loops
• Expand project UI is gated on hasSpec to prevent opening the expand modal when no spec exists

Changes

Server (5 files):

• expand_project.py - Accept before validation + spec check
• spec_creation.py - Accept before validation, fix validate_project_name usage
• assistant_chat.py - Accept before validation, fix validate_project_name usage
• terminal.py - Accept before all validation checks
• websocket.py - Accept before validation, ConnectionManager.connect() no longer double-accepts

Client (6 files):

• useWebSocket.ts / useExpandChat.ts / useSpecChat.ts - Skip reconnection on 4xxx close codes
• App.tsx - Gate expand shortcut (E key) and modal render on hasSpec
• KanbanBoard.tsx - Gate expand button on hasSpec
• KeyboardShortcutsHelp.tsx - Update E key context text

Test plan

• Open existing project without spec → no 403 errors in server logs
• Expand button hidden when hasSpec is false
• E key does nothing when hasSpec is false
• WebSocket errors show meaningful JSON messages instead of 403
• No retry loops on application-level close codes

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