feat(studio): AI chat pane with BYOK and cost counter

[pbertsch]

Summary

• ✦ AI Chat button in Studio toolbar toggles a bottom panel (⌘⇧A shortcut)
• BYOK only: user provides their own Anthropic API key (no key bundled)
• Platform keychain storage: macOS Keychain / Windows Credential Manager / Linux libsecret via zalando/go-keyring — key never written to disk or config files
• Model: claude-sonnet-4-6 (default, latest Sonnet)
• Multi-turn chat: full conversation history sent per turn; system prompt includes the current file content as context
• Cost counter: running total of input + output tokens + estimated USD (Sonnet 4.6 rates: $3/$15 per 1M tokens) shown in pane title bar
• API key overlay: 🔑 button opens a settings overlay, key saved/removed from keychain, shows masked input
• Layout: chat panel is a 260px strip below the grid, doesn't disrupt existing panes

Dependencies added (studio/go.mod)

• github.com/zalando/go-keyring v0.2.8
• github.com/danieljoos/wincred v1.2.3 (Windows backend, transitive)
• github.com/godbus/dbus/v5 v5.2.2 (Linux backend, upgraded)

Test plan

• go build ./... passes (root repo)
• cd studio && go build . passes
• Frontend: npm run build in studio/frontend/ passes
• Click ✦ button → chat pane slides in at bottom; click again → hides
• ⌘⇧A toggles chat
• Click 🔑 → API key overlay opens; enter key → saved to macOS Keychain (verify via Keychain Access.app)
• Send a message about the open .probe file → assistant replies with ProbeScript context
• Cost counter updates after each turn (input tokens / output tokens / $)
• Clear button resets history and cost counter
• Invalid key → clear error toast "invalid API key"
• No key stored → error toast "no API key stored"