T21a: Documents, editing

- markdown read with pulldown-cmark: marks for live styling, blocks, links, words
- forms::Highlight gains weight, slant and strike; measure and paint share runs
- Inter Italic embedded, so emphasis reads slanted
- forms::mention's popup and keys become Suggestions, shared with the editors
- RichTextEditor with FixedFormatToolbar, the floating toolbar, LinkEditor,
  @ and : suggestions, WordCount and ReadingTime
- MarkdownEditor: visual (a pressed block opens as source), source, split
- BlockEditor: prose, lists, to-dos, quotes, callouts, toggles, code, math,
  diagrams the app draws, dividers, tables that grow and merge, pictures to
  resize and align, cards for video and pages, columns, synced text; slash menu,
  markdown shortcuts, handles that drag and open a menu, one undo for all
- TodoItem and Checklist
- icons: heading-1..3, workflow, merge
- gallery: the Documents page with scripted states

assets/icons/heading-1.svg

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+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-heading-1"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="M4 12h8" />
+  <path d="M4 18V6" />
+  <path d="M12 18V6" />
+  <path d="m17 12 3-2v8" />
+</svg>

assets/icons/heading-2.svg

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+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-heading-2"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="M4 12h8" />
+  <path d="M4 18V6" />
+  <path d="M12 18V6" />
+  <path d="M21 18h-4c0-4 4-3 4-6 0-1.5-2-2.5-4-1" />
+</svg>

assets/icons/heading-3.svg

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+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-heading-3"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="M4 12h8" />
+  <path d="M4 18V6" />
+  <path d="M12 18V6" />
+  <path d="M17.5 10.5c1.7-1 3.5 0 3.5 1.5a2 2 0 0 1-2 2" />
+  <path d="M17 17.5c2 1.5 4 .3 4-1.5a2 2 0 0 0-2-2" />
+</svg>

assets/icons/merge.svg

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+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-merge"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="m8 6 4-4 4 4" />
+  <path d="M12 2v10.3a4 4 0 0 1-1.172 2.872L4 22" />
+  <path d="m20 22-5-5" />
+</svg>

assets/icons/workflow.svg

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+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-workflow"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <rect width="8" height="8" x="3" y="3" rx="2" />
+  <path d="M7 11v4a2 2 0 0 0 2 2h4" />
+  <rect width="8" height="8" x="13" y="13" rx="2" />
+</svg>