feat(share): custom /l/:alias pretty links (share_aliases table) (#205)

Add a retargetable, human-readable vanity link namespace /l/<alias> that
sits alongside the untouched /share/... routes.

- New share_aliases table (workspace-scoped, UNIQUE(workspace_id, alias),
  page_id nullable ON DELETE SET NULL so the address outlives its target).
- ShareAliasRepo + ShareAliasService (create / no-op / 409 reassign guard /
  availability / request-time readable-target resolution through the single
  existing share boundary).
- Public ShareAliasRedirectController (GET /l/:alias) issues a 302 (never 301,
  the target is mutable) to the canonical /share/:key/p/:slug page; unknown /
  dangling / no-longer-readable aliases serve the SPA index with no leak.
  'l/:alias' excluded from the global /api prefix.
- Authenticated ShareAliasController (set/remove/availability/for-page).
- Shared ASCII-only normalize/validate util (server + client copies).
- Client: Custom address block in the share modal (live normalize + debounced
  availability + copy + reassign confirmation dialog).
- Unit tests: util, repo SQL-shape, service semantics, migration/entity sanity
  (server jest) + client alias util (vitest).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/share/share-alias.util.ts

@@ -0,0 +1,26 @@
+/**
+ * Client copy of the vanity share-alias helpers. Kept in sync with the server
+ * (`apps/server/src/core/share/share-alias.util.ts`) so live input feedback
+ * matches what the server will store/accept. ASCII-only, lowercase, hyphen
+ * separated, length 2..60.
+ */
+
+// Normalize a user-provided vanity alias into canonical ASCII storage form.
+export function normalizeShareAlias(raw: string): string {
+  return (raw ?? "")
+    .trim()
+    .toLowerCase()
+    .replace(/[\s_]+/g, "-")
+    .replace(/-{2,}/g, "-")
+    .replace(/^-+|-+$/g, "");
+}
+
+const ALIAS_RE = /^[a-z0-9]+(?:-[a-z0-9]+)*$/;
+export function isValidShareAlias(alias: string): boolean {
+  return (
+    typeof alias === "string" &&
+    alias.length >= 2 &&
+    alias.length <= 60 &&
+    ALIAS_RE.test(alias)
+  );
+}