feat(workspace): numbered lists and reliable markdown paste

- block type 'numbered' (/number): 1., a., i. by nesting, continues on
  Enter, an empty item ends the list; markdown export/import as 1.
- pasting: parser moved to lib/pasteOutline with tests; blank lines split
  paragraphs, numbered lists keep their type, indentation levels follow
  the widths in use (2, 3 or 4 spaces, tabs)
- text that stays in the focused block after pasting (a paragraph with
  line breaks) was dropped; the block now takes it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/outlineList.ts

@@ -0,0 +1,65 @@
+import type { DocumentBlock } from './types';
+
+// Numbered lists in the outliner: consecutive 'numbered' siblings count 1, 2, 3 …; a numbered list
+// inside another one uses a, b, c, the next level i, ii, iii, then numbers again.
+
+function alpha(n: number): string {
+  let s = '';
+  while (n > 0) {
+    n--;
+    s = String.fromCharCode(97 + (n % 26)) + s;
+    n = Math.floor(n / 26);
+  }
+  return s;
+}
+
+const ROMAN: [number, string][] = [
+  [1000, 'm'], [900, 'cm'], [500, 'd'], [400, 'cd'], [100, 'c'], [90, 'xc'],
+  [50, 'l'], [40, 'xl'], [10, 'x'], [9, 'ix'], [5, 'v'], [4, 'iv'], [1, 'i'],
+];
+
+function roman(n: number): string {
+  let s = '';
+  for (const [v, r] of ROMAN) {
+    while (n >= v) {
+      s += r;
+      n -= v;
+    }
+  }
+  return s;
+}
+
+export function listLabel(n: number, depth: number): string {
+  switch (depth % 3) {
+    case 1:
+      return `${alpha(n)}.`;
+    case 2:
+      return `${roman(n)}.`;
+    default:
+      return `${n}.`;
+  }
+}
+
+/** Label ("1.", "b.", "iii.") for every numbered block, in document order. */
+export function numberedLabels(blocks: DocumentBlock[]): Map<string, string> {
+  const labels = new Map<string, string>();
+  const runs = new Map<number, number>(); // level -> count of the current run
+  const chain: DocumentBlock[] = []; // last block seen per level = ancestors of the next one
+  for (const b of blocks) {
+    const level = Math.max(0, b.level || 0);
+    for (const l of Array.from(runs.keys())) {
+      if (l > level) runs.delete(l);
+    }
+    chain.length = Math.min(chain.length, level);
+    if (b.type === 'numbered') {
+      const n = (runs.get(level) ?? 0) + 1;
+      runs.set(level, n);
+      const depth = chain.filter((a) => a && a.type === 'numbered').length;
+      labels.set(b.id, listLabel(n, depth));
+    } else {
+      runs.delete(level);
+    }
+    chain[level] = b;
+  }
+  return labels;
+}