feat: prompt for API key when server requires authentication

[ajainvivek]

Summary

• Servers with auth enabled return 401/403 on unauthenticated requests. Previously the client silently failed or showed a generic connection error with no way to enter credentials.
• This PR detects auth-required servers and surfaces a focused API key prompt dialog before completing the connection.

Changes

src/lib/api.ts

• checkAuth() — probes /v1/tables without credentials; returns { authRequired: true } on 401/403, gracefully ignores network errors so it never blocks the connect flow
• updateApiKey() — applies a key to an existing in-memory client instance after the prompt is confirmed

src/components/connection/ApiKeyPromptDialog.tsx (new)

• Modal dialog with a password input, loading state, and inline error feedback
• Validates the key by calling listTables() before completing — shows "Invalid API key" if the server rejects it

src/components/layout/Sidebar.tsx

• handleConnect now probes for auth after a successful connection test
• If auth is required: parks the client in a ref, sets pendingAuthConnection, shows the dialog
• handleApiKeySubmit applies the key, calls listTables() to validate, then finalises setClient / setActiveConnection

src/App.tsx

• On startup, if a persisted connection has no stored apiKey and the server now requires one, drops activeConnection back to null so the user lands on the connection list instead of a broken session

Test plan

• Connect to a server with auth disabled — flow unchanged, no dialog shown
• Connect to a server with auth enabled and no stored key — API key dialog appears
• Enter a wrong key — "Invalid API key" error shown, dialog stays open
• Enter the correct key — connects successfully
• Restart the app with a previously connected auth-required server that has no stored key — redirected to connection list