feat: blend title bar, remove menu, add agent activity indicators

- Configure hidden title bar with titleBarOverlay matching dark theme
- Remove native menu bar via Menu.setApplicationMenu(null)
- Fix backgroundColor mismatch (#0f172a -> #09090b)
- Add full-width drag region in AppShell for window dragging
- Adjust sidebar and chat headers with top padding for overlay zone
- Add ThinkingDots component with animated dot sequence
- Show activity indicator in chat when agent is processing
- Replace 'Generating...' spinner with inline blinking cursor
- Refine header status badge to subtle pulsing dot
- Add BACKEND_UI_CHANGES.md documenting future sidecar protocol additions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

BACKEND_UI_CHANGES.md

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+# Backend UI Changes
+
+This document describes changes to the .NET sidecar / backend protocol that would enable richer agent-activity reporting in the chat UI. These are **not yet implemented** — the current UI infers activity state from existing events. A separate agent should implement these changes.
+
+## Context
+
+The chat UI now shows a "Thinking…" indicator while the agent processes a request (before streaming starts) and a blinking cursor while the response streams in. These states are inferred from the existing `status` and `message-delta` session events.
+
+To display more granular activity (e.g. "Using tool X…", "Agent Y is thinking…", "Handing off to Agent Z…"), the sidecar protocol needs a new event kind.
+
+## Proposed protocol addition
+
+### New event kind: `agent-activity`
+
+Add a new value `'agent-activity'` to `SessionEventKind` in `src/shared/domain/event.ts`:
+
+```typescript
+export type SessionEventKind =
+  | 'status'
+  | 'message-delta'
+  | 'message-complete'
+  | 'agent-activity'   // ← new
+  | 'error';
+```
+
+### New fields on `SessionEventRecord`
+
+```typescript
+export interface SessionEventRecord {
+  sessionId: string;
+  kind: SessionEventKind;
+  occurredAt: string;
+
+  // Existing fields…
+  status?: 'idle' | 'running' | 'error';
+  messageId?: string;
+  authorName?: string;
+  contentDelta?: string;
+  error?: string;
+
+  // New fields for 'agent-activity' events
+  activityType?: 'thinking' | 'tool-calling' | 'handoff' | 'completed';
+  agentName?: string;
+  toolName?: string;
+}
+```
+
+### Sidecar event mapping
+
+The .NET sidecar should emit `agent-activity` events at these points:
+
+| MAF lifecycle point | `activityType` | `agentName` | `toolName` |
+|---|---|---|---|
+| Agent begins processing a turn | `thinking` | agent name | — |
+| Agent invokes a tool | `tool-calling` | agent name | tool name |
+| Handoff orchestration transfers control | `handoff` | target agent name | — |
+| Agent finishes its contribution | `completed` | agent name | — |
+
+### .NET sidecar changes
+
+In the sidecar's turn-execution pipeline, emit a new JSON event type alongside the existing `turn-delta` and `turn-complete`:
+
+```json
+{
+  "type": "agent-activity",
+  "requestId": "…",
+  "sessionId": "…",
+  "activityType": "tool-calling",
+  "agentName": "Code Reviewer",
+  "toolName": "read_file"
+}
+```
+
+The Electron main process (`KopayaAppService`) should map this to a `SessionEventRecord` with `kind: 'agent-activity'` and forward it to the renderer via the existing `sessions:event` channel.
+
+### Renderer consumption (already prepared)
+
+Once these events are available, the `ChatPane` activity indicator can be enhanced to show contextual messages like:
+
+- "Code Reviewer is thinking…"
+- "Code Reviewer is using read_file…"
+- "Handing off to Summarizer…"
+
+The `ThinkingDots` component and activity indicator section in `ChatPane.tsx` are designed to be extended with this data.
+
+## Files to change
+
+| Layer | File | Change |
+|---|---|---|
+| Shared | `src/shared/domain/event.ts` | Add `'agent-activity'` to `SessionEventKind`, add optional `activityType` / `agentName` / `toolName` fields |
+| Main | `src/main/sidecar/sidecar.ts` | Parse `agent-activity` events from sidecar JSON output |
+| Main | `src/main/KopayaAppService.ts` | Map parsed activity events to `SessionEventRecord` and emit via `session-event` |
+| Sidecar | `sidecar/src/Kopaya.AgentHost/…` | Emit `agent-activity` JSON events during MAF turn execution |