Adds an in-page element picker to the preview browser. Clicking the
crosshair button in the chrome row activates a blue-highlight picker
inside the guest webview; clicking an element captures its component
name (via react-grab), source location, html/css preview, and selector,
then attaches it to the chat composer as a chip that serializes into an
`<element_context>` block in the outgoing message.

Architecture:
- Per-`<webview>` preload bundle (`preview-pick-preload.cjs`) renders
  the overlay, hosts the picker event loop, and bubbles the picked
  payload back to main via the per-WebContents `wc.ipc` channel (not
  `sendToHost`, which only fires on the host renderer's <webview>
  element and never reaches main).
- Main coordinates via `PreviewViewManager.pickElement(tabId)`, which
  cancels any in-flight session, force-focuses the guest (so the first
  click on a remote page actually reaches the preload), then awaits the
  payload. User-initiated cancels (Escape, beforeunload) echo `null`
  back to main; main-initiated cancels and supersession tear down
  silently to avoid the new-pick-resolves-with-stale-null race.
- Renderer fetches partition + webPreferences + preload URL in a single
  `getPreviewConfig()` IPC call, snapshots the previously-focused host
  element before triggering a pick, and restores focus when the pick
  resolves so the user's textarea cursor isn't lost.

Security posture for the guest webview:
- `webpreferences="contextIsolation=false,sandbox=true,nodeIntegration=false"`
  centralized in `preview-webview-preferences.ts`. contextIsolation off
  is required so react-grab's `getElementContext` can reach the page's
  React DevTools hook on `globalThis`. sandbox stays on so the page
  cannot reach Node APIs even with shared globals (without it, the
  preload's `require` would land on the page's `globalThis` and any
  third-party site could send arbitrary IPC to main).
- Defense in depth: a `will-attach-webview` handler in main, gated on
  the preview partition, force-pins `sandbox: true`, all
  `nodeIntegration*: false`, and the absolute preload PATH (not URL —
  that field rejects file:// URLs with "preload script must have
  absolute path" and silently disables the picker).

Composer + transcript integration:
- New `elementContexts` slice in `composerDraftStore` (mirrors the
  terminal-context slice: dedup by selector+tag+component+url, persist
  via partializer, restore on send-failure retry).
- `ComposerPendingElementContexts` chip row above the editor.
- `deriveDisplayedUserMessageState` now strips both `<element_context>`
  AND `<terminal_context>` blocks (element first, since it's appended
  last) and exposes element entries to `MessagesTimeline`, which renders
  them as compact chips beneath the message body.
- Pick button is disabled with explanatory tooltip when the page failed
  to load (the React `<PreviewUnreachable>` overlay covers the webview,
  so picks would silently dangle otherwise).

Tests added:
- `preview-webview-preferences.test.ts` locks down the security flags
  (contextIsolation=false, sandbox=true, nodeIntegration=false, no
  whitespace, only true/false literal values).
- `preview-pick-label-position.test.ts` covers the floating-label
  clamp/flip math (no off-screen overflow, flip-below when no room
  above, etc.).
- `picked-element-payload.test.ts` validator coverage.
- `elementContext.test.ts` for the serialization round-trip,
  normalization, dedup, and label formatting.
- `composerDraftStore.test.ts` element-contexts slice (add, dedup,
  remove, set, clear, persistence round-trip).
- `ChatView.logic.test.ts` sendable-content-with-element-only.

Build: new `tsdown` entry inlines react-grab + bippy into the picker
preload bundle (~59KB / 19KB gzipped).