feat(core): edit icon in navigation panel (#13595)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Rename dialog now edits per-item explorer icons (emoji or custom) and
can skip name-change callbacks. Doc icon picker added to the editor with
localized "Add icon" placeholder and readonly rendering. Icon editor
supports fallbacks, trigger variants, and improved input/test-id wiring.

- **Style**
- Updated icon picker and trigger sizing and placeholder visuals;
title/icon layout adjustments.

- **Chores**
- Explorer icon storage and module added to persist and serve icons
across the app.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/i18n/src/i18n.gen.ts

@@ -8314,6 +8314,10 @@ export function useAFFiNEI18N(): {
       * `Cut`
       */
     ["com.affine.context-menu.cut"](): string;
+    /**
+      * `Add icon`
+      */
+    ["com.affine.docIconPicker.placeholder"](): string;
     /**
       * `An internal error occurred.`
       */

packages/frontend/i18n/src/resources/en.json

@@ -2086,6 +2086,7 @@
   "com.affine.context-menu.copy": "Copy",
   "com.affine.context-menu.paste": "Paste",
   "com.affine.context-menu.cut": "Cut",
+  "com.affine.docIconPicker.placeholder": "Add icon",
   "error.INTERNAL_SERVER_ERROR": "An internal error occurred.",
   "error.NETWORK_ERROR": "Network error.",
   "error.TOO_MANY_REQUEST": "Too many requests.",