Article editor toolbar: toggling fences/links, class pickers, sizes, Tab indent

- Image insert always adds an empty "" caption with the cursor inside.
- Fenced blocks (``` code, ::: aside) share one toggle: clicked inside
  one it is removed and the content selected; otherwise the selection
  (expanded to whole lines) is wrapped, cursor left on the opener line.
- Code button: inline wrap toggles (selection preserved, backtick runs),
  line-spanning selections make fenced blocks.
- Link button toggles: clicked inside [label](url) it unwraps.
- Block classes via two pickers (placement, AA size) with current-class
  indication and a "normal" reset; placement replaces ::: container
  names, fences get their own attribute line.
- New .small/.large/.huge text-size classes (0.7/1.5/3em); base body is
  exactly 1rem so the scale is uniform.
- .left/.right floats generalized from figures to any block.
- Toolbar polish: non-emoji glyphs, scaled-up symbols, borderless
  hover/active states, reordered (B/i and size last).
- Editor panel no longer closes on Escape.
- CodeMirror editors capture Tab/Shift-Tab for indent/dedent.

frontend/src/SiteEditor.vue

@@ -5,6 +5,8 @@
 import { computed, onActivated, onMounted, onUnmounted, ref, watch } from 'vue'
 import { EditorView, basicSetup } from 'codemirror'
 import { EditorState } from '@codemirror/state'
+import { keymap } from '@codemirror/view'
+import { indentWithTab } from '@codemirror/commands'
 import { css } from '@codemirror/lang-css'
 import { html } from '@codemirror/lang-html'
 import { cmHighlight, cmTheme } from './cmtheme'
@@ -487,6 +489,8 @@ onMounted(async () => {
       doc: '',
       extensions: [
         basicSetup,
+        // Tab/Shift-Tab indent and dedent instead of moving focus.
+        keymap.of([indentWithTab]),
         css(),
         cmTheme,
         cmHighlight,
@@ -506,6 +510,8 @@ onMounted(async () => {
       doc: '',
       extensions: [
         basicSetup,
+        // Tab/Shift-Tab indent and dedent instead of moving focus.
+        keymap.of([indentWithTab]),
         html(),
         cmTheme,
         cmHighlight,