fix: gate BranchPicker and edit pencil on runtime capabilities

[pikann]

Summary

• The BranchPicker and the user message's edit pencil in thread.tsx rendered unconditionally, regardless of whether the runtime actually supports switching branches or editing — assistant-ui's underlying hooks only check local UI state, not thread.capabilities.
• Neither conversation-view.tsx nor ai-chat-float.tsx wire up onEdit/setMessages on useExternalStoreRuntime, so those capabilities are always false there — yet the controls showed up anyway on every message, and clicking them would throw a runtime error ("Runtime does not support switching branches" / "...editing messages").
• Wrapped both in <AuiIf condition={(s) => s.thread.capabilities.switchToBranch}> and <AuiIf condition={(s) => s.thread.capabilities.edit}> so they only render when a runtime genuinely supports them.

Fixes #313

Test plan

• npx vitest run in apps/web — 315 tests passing
• npx tsc -b — no type errors
• Verified via a runtime harness that capabilities.edit and capabilities.switchToBranch are false for the exact useExternalStoreRuntime config used in conversation-view.tsx / ai-chat-float.tsx

🤖 Generated with Claude Code

[pullfrog]

✅ No new issues found.

Reviewed changes — gates BranchPicker and the edit pencil in thread.tsx behind thread capability checks so they don't appear (and throw) when the runtime doesn't support those operations.

• Gate edit pencil on capabilities.edit — ActionBarPrimitive.Edit is now wrapped in <AuiIf> so it only renders when the runtime has onEdit wired up.
• Gate BranchPicker on capabilities.switchToBranch — BranchPickerPrimitive.Root is now wrapped in <AuiIf> so it only renders when the runtime has setMessages wired up.

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