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/FileTree.tsx

@@ -1,5 +1,11 @@
 import { Component, createMemo, createSignal, Show } from "solid-js";
 import { useLocation } from "@solidjs/router";
+import {
+  TbOutlineFolderOpen,
+  TbOutlineFolder,
+  TbOutlineFile,
+  TbOutlineChevronRight,
+} from "solid-icons/tb";
 import { type FileNode, type TocNode } from "../data-loader";
 import { useNavigateWithParams } from "./useNavigateWithParams";
 import { useScrollContainer } from "../App";
@@ -61,10 +67,14 @@ export const FileTreeNode: Component<{
         onClick={handleClick}
       >
         <Show when={isDir}>
-          <span class="mr-1 text-gray-400">{isExpanded() ? "📂" : "📁"}</span>
+          {isExpanded() ? (
+            <TbOutlineFolderOpen class="mr-1 text-gray-400" />
+          ) : (
+            <TbOutlineFolder class="mr-1 text-gray-400" />
+          )}
         </Show>
         <Show when={!isDir}>
-          <span class="mr-1 text-gray-400">📄</span>
+          <TbOutlineFile class="mr-1 text-gray-400" />
         </Show>
         <span class="text-sm truncate">{props.node.name}</span>
       </a>
@@ -151,7 +161,7 @@ export const HeadingNode: Component<{
           <span
             class={`${hasChildren ? "" : "invisible"} ${isExpanded() ? "rotate-90" : ""} mt-1 inline-block transition-transform `}
           >
-            ▶
+            <TbOutlineChevronRight class="inline-block align-[-0.125em]" />
           </span>
         </span>
         <a