Centralize platform gating and fix iPad/tablet support

Add `packages/app/src/constants/platform.ts` with canonical gates
(isWeb, isNative, getIsElectron) and refactor all ~100 ad-hoc
Platform.OS checks across 69 files to use shared imports.

Fix sidebar hover crash on native (onPointerEnter → onHoverIn),
fix tooltip to use breakpoint instead of platform detection, make
sidebar action buttons always visible on native where hover doesn't
work, and document the platform gating decision matrix in CLAUDE.md.

CLAUDE.md

@@ -55,6 +55,46 @@ See [docs/DEVELOPMENT.md](docs/DEVELOPMENT.md) for full setup, build sync requir
   - Never narrow a field's type (e.g. `string` → `enum`, `nullable` → non-null).
   - Test with: "does a 6-month-old client still parse this?" and "does a 6-month-old daemon still send something this client accepts?"
 
+## Platform gating
+
+The app runs on iOS, Android, web (browser), and web (Electron desktop). Code is cross-platform by default. Gate only when you must. Import gates from `@/constants/platform`.
+
+### The four gates
+
+| Gate | Type | When to use |
+|---|---|---|
+| `isWeb` | constant | DOM APIs — `document`, `window`, `<div>`, `addEventListener`, `ResizeObserver`. This is the **exception**, not the default. |
+| `isNative` | constant | Native-only APIs — Haptics, `StatusBar.currentHeight`, push tokens, camera/scanner, `expo-av`. |
+| `getIsElectron()` | cached fn | Desktop wrapper features — file dialogs, titlebar drag region, daemon management, app updates, dock badges. |
+| `useIsCompactFormFactor()` | hook | Layout decisions — sidebar overlay vs pinned, modal vs full screen, single-panel vs split. From `@/constants/layout`. |
+
+### Decision matrix
+
+| I need to... | Use |
+|---|---|
+| Access DOM (`document`, `window`, `<div>`, `addEventListener`) | `if (isWeb)` |
+| Use a native-only API (Haptics, push tokens, camera) | `if (isNative)` |
+| Use an Electron bridge (file dialog, titlebar, updates) | `if (getIsElectron())` |
+| Switch layout between phone and tablet/desktop | `useIsCompactFormFactor()` |
+| Show something on hover, always-visible on native | `isHovered \|\| isNative \|\| isCompact` (hover only works on web) |
+| Gate to iOS or Android specifically | `Platform.OS === "ios"` / `Platform.OS === "android"` (rare, keep inline) |
+
+### Rules
+
+- **Default is cross-platform.** Don't gate unless you have a specific reason.
+- **Prefer Metro file extensions over `if` statements.** When a module has fundamentally different implementations per platform, use `.web.ts` / `.native.ts` file extensions instead of runtime `if (isWeb)` branches. Metro resolves the correct file at build time — the unused platform code is never bundled. Reserve `if (isWeb)` for small, inline checks (a single line or a few props). If you find yourself writing a large `if (isWeb) { ... } else { ... }` block, split into separate files instead.
+  ```
+  hooks/
+    use-audio-recorder.web.ts    ← uses Web Audio API
+    use-audio-recorder.native.ts ← uses expo-audio
+  ```
+  Import as `@/hooks/use-audio-recorder` — Metro picks the right file automatically.
+- **NEVER use raw DOM APIs without `isWeb` guard.** DOM APIs crash native. Casting a RN ref to `HTMLElement` is a red flag — ensure the block is web-only.
+- **NEVER use `onPointerEnter`/`onPointerLeave`.** They don't fire on native iOS.
+- **Hover only works on web.** React Native's `onHoverIn`/`onHoverOut` on `Pressable` does NOT fire on native iOS/iPad — the underlying W3C pointer events are behind disabled experimental flags. For hover-to-show UI (kebab menus, action buttons), use `isHovered || isNative || isCompact` so the controls are always visible on native and hover-to-show on web.
+- **Don't use Platform.OS as a proxy for layout capabilities.** Use breakpoints for layout decisions, not platform checks.
+- **Import `isWeb`/`isNative` from `@/constants/platform`.** Never write `const isWeb = Platform.OS === "web"` locally.
+
 ## Debugging