Fixes batch 3: autosave, campaign-agnostic PCs, live push, builder

- Autosave: while signed in, a debounced full backup pushes whenever durable data
  changes (useCloudAutosave watches a fingerprint of the main tables; high-churn
  tables excluded). No-op signed out.
- Campaign-agnostic characters: PCs are now global — the Characters roster shows
  every player character regardless of active campaign, and any PC can be added to
  a fight. NPCs stay per-campaign. (charactersRepo.listAllPcs / useAllPcs; difficulty
  fallback stays campaign-scoped.)
- Live "bring your own character": a joining player can push one of their own local
  characters; the GM's grant inserts it into the campaign and seats them.
- Character builder: a system picker (5e / PF2e, defaults to the campaign) so creation
  no longer assumes 5e; PF2e skill wording ("Trained" vs "proficient"); spell step now
  explains how many to pick + that they're known spells prepared later.

230 unit + 34 e2e + 2 realtime green.

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

src/lib/db/repositories.ts

@@ -88,6 +88,11 @@ export const charactersRepo = {
     return db.characters.where('campaignId').equals(campaignId).toArray();
   },
 
+  /** All player characters across every campaign (PCs are campaign-agnostic). */
+  listAllPcs(): Promise<Character[]> {
+    return db.characters.where('kind').equals('pc').toArray();
+  },
+
   get(id: string): Promise<Character | undefined> {
     return db.characters.get(id);
   },