fix(electrobun): address all 22 Codex review #2 findings

CRITICAL:
- DocsTab XSS: DOMPurify sanitization on all {@html} output
- File RPC path traversal: guardPath() validates against project CWDs

HIGH:
- SSH injection: spawn /usr/bin/ssh via PTY args, no shell string
- Search XSS: strip HTML, highlight matches client-side with <mark>
- Terminal listener leak: cleanup functions stored + called in onDestroy
- FileBrowser race: request token, discard stale responses
- SearchOverlay race: same request token pattern
- App startup ordering: groups.list chains into active_group restore
- PtyClient timeout: 5-second auth timeout on connect()
- Rule 55: 6 {#if} patterns converted to style:display toggle

MEDIUM:
- Agent persistence: only persist NEW messages (lastPersistedIndex)
- Search errors: typed error response, "Invalid query" UI
- Health store wired: agent events call recordActivity/setProjectStatus
- index.ts SRP: split into 8 domain handler modules (298 lines)
- App.svelte: extracted workspace-store.svelte.ts
- rpc.ts: typed AppRpcHandle, removed `any`

LOW:
- CommandPalette listener wired in App.svelte
- Dead code removed (removeGroup, onDragStart, plugin loaded)

ui-electrobun/src/mainview/ChatInput.svelte

@@ -242,15 +242,14 @@
   </div>
 </div>
 
-<!-- Click outside overlay to close popup -->
-{#if openPopup !== null}
-  <!-- svelte-ignore a11y_no_static_element_interactions -->
-  <div
-    class="popup-backdrop"
-    onclick={closePopup}
-    onkeydown={e => e.key === 'Escape' && closePopup()}
-  ></div>
-{/if}
+<!-- Fix #11: display toggle for popup backdrop -->
+<!-- svelte-ignore a11y_no_static_element_interactions -->
+<div
+  class="popup-backdrop"
+  style:display={openPopup !== null ? 'block' : 'none'}
+  onclick={closePopup}
+  onkeydown={e => e.key === 'Escape' && closePopup()}
+></div>
 
 <style>
   .chat-input-outer {