feat: add Zustand stores for focus, navigation, capture, and batch

Implements the frontend state management layer using Zustand. Each store
is single-purpose and testable in isolation.

Focus Store (focus-store.ts):
- Tracks current focused item (THE ONE THING)
- Manages the queue of remaining items
- Actions: setItems, act (start/defer/skip), setFocus, reorderQueue
- Advancing through items removes from queue, promotes next to current

Navigation Store (nav-store.ts):
- Simple view routing: focus | queue | inbox
- No URL-based routing needed for native app
- Default view is "focus"

Capture Store (capture-store.ts):
- Manages quick capture overlay state
- Tracks submission status and errors
- Opens via global shortcut event listener

Batch Store (batch-store.ts):
- Manages batch processing mode for rapid item completion
- Tracks items, their statuses (pending/done/skipped), and current index
- Derives counts: completedCount, skippedCount, isFinished
- Used for "knock out all code reviews" workflow

State design principles:
- No derived state stored; computed on access
- Actions are pure mutations with logging
- Loading/error states colocated with data

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/stores/capture-store.ts

@@ -0,0 +1,55 @@
+/**
+ * Quick Capture store.
+ *
+ * Manages the overlay state for the global Cmd+Shift+C capture flow.
+ */
+
+import { create } from "zustand";
+
+interface CaptureState {
+  isOpen: boolean;
+  isSubmitting: boolean;
+  lastCapturedId: string | null;
+  error: string | null;
+}
+
+interface CaptureActions {
+  open: () => void;
+  close: () => void;
+  setSubmitting: (submitting: boolean) => void;
+  captureSuccess: (beadId: string) => void;
+  captureError: (message: string) => void;
+}
+
+export const useCaptureStore = create<CaptureState & CaptureActions>((set) => ({
+  isOpen: false,
+  isSubmitting: false,
+  lastCapturedId: null,
+  error: null,
+
+  open: () => set({ isOpen: true, error: null }),
+
+  close: () =>
+    set({
+      isOpen: false,
+      isSubmitting: false,
+      error: null,
+    }),
+
+  setSubmitting: (submitting) => set({ isSubmitting: submitting }),
+
+  captureSuccess: (beadId) =>
+    set({
+      lastCapturedId: beadId,
+      isSubmitting: false,
+      isOpen: false,
+      error: null,
+    }),
+
+  captureError: (message) =>
+    set({
+      error: message,
+      isSubmitting: false,
+      isOpen: true,
+    }),
+}));