feat(ui): replace unicode icons with solid-icons

Swap emoji and unicode glyphs for Tabler components so icons inherit
currentColor and font-size and render consistently across platforms.

- add src/components/doc-icons.ts to resolve doc-entry frontmatter icon
  keys to components; frontmatter now stores semantic keys
- replace the align-*.png assets with TbOutlineAlign* icons
- render RevealManager action SVGs as components instead of injected
  innerHTML strings
- carry completion icons via an optional CompletionItem.icon field

src/components/journal/CreateSessionDialog.tsx

@@ -2,6 +2,7 @@
  * CreateSessionDialog — modal for naming a new session
  */
 import { Component, createSignal, onMount } from "solid-js";
+import { TbOutlineX } from "solid-icons/tb";
 import { createSession, joinSession } from "../stores/journalStream";
 
 interface CreateSessionDialogProps {
@@ -48,7 +49,7 @@ export const CreateSessionDialog: Component<CreateSessionDialogProps> = (
             onClick={props.onClose}
             class="text-gray-400 hover:text-gray-600 text-sm"
           >
-            ✕
+            <TbOutlineX />
           </button>
         </div>
         <p class="text-xs text-gray-500">输入新会话的名称。</p>