TipTap editor, version history, search, attachments — and a cleanup

The app is meant to run in daily private use now, so this pass was mostly
about the ways it could quietly lose data.

Reliability and security:
- the server no longer invents a JWT_SECRET or ENCRYPTION_KEY when one is
  missing; it refuses to start. The old fallback silently made every stored
  note unreadable after a restart.
- notes are encrypted with AES-256-GCM (v2:); existing CBC bodies still read.
- login is rate limited and compares against a dummy hash when the user does
  not exist, so timing does not reveal who is registered.
- a server response issued before a local save is never adopted. This had
  already eaten drawing strokes and note links; sync.hasPendingWrites() now
  guards every path that takes a body from the server.

Editor and features:
- TipTap replaces the contenteditable editor: tables, task lists, and links
  to other notes and notebooks, with a backlinks bar on the target.
- content format v2 ({v:2,doc}); v1 {html,tasks} bodies migrate on open.
- version history: the state before each write is snapshotted, coalesced
  into 5-minute windows, 50 versions or 90 days per note.
- full-text search runs in the client, because the server cannot read
  encrypted bodies and a server index would defeat the encryption.
- image attachments are stored by SHA-256 and deduplicated; the client
  fetches them authenticated and caches the blob, so they work offline.
- drawings are vector strokes in a fixed 1200x1600 page space and now look
  the same on every screen (0.0-0.2% deviation across four widths).
- login is by username; existing email accounts migrate automatically.

Operations:
- data lives on host bind mounts, not Docker volumes, so Proxmox Backup
  covers it. Back up .env with it — without ENCRYPTION_KEY the dump is
  unreadable.

Structure:
- NoteApp.jsx is gone. App.jsx now holds only where the user is; note data,
  search, layout, history and drag each live behind a hook, and the four
  panes are stateless.
- the whole data layer is TypeScript, with 81 unit tests concentrated on
  the places that have actually broken.
- fixed along the way: the browser back button had a dead press, because
  restoring a history entry pushed a duplicate one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/hooks/usePwaInstall.ts

@@ -0,0 +1,41 @@
+import { useEffect, useState } from "react";
+
+/** Chrome's non-standard install event. Not in lib.dom, so it is declared here. */
+interface BeforeInstallPromptEvent extends Event {
+  prompt(): Promise<void>;
+  userChoice: Promise<{ outcome: "accepted" | "dismissed" }>;
+}
+
+export interface PwaInstall {
+  /** Non-null only while the browser is willing to show an install prompt. */
+  canInstall: boolean;
+  promptInstall: () => Promise<void>;
+}
+
+/**
+ * Captures the browser's install offer so the app can present it at a sensible
+ * moment instead of letting the browser choose one.
+ */
+export function usePwaInstall(): PwaInstall {
+  const [event, setEvent] = useState<BeforeInstallPromptEvent | null>(null);
+
+  useEffect(() => {
+    const onPrompt = (e: Event) => {
+      e.preventDefault();
+      setEvent(e as BeforeInstallPromptEvent);
+    };
+    window.addEventListener("beforeinstallprompt", onPrompt);
+    return () => window.removeEventListener("beforeinstallprompt", onPrompt);
+  }, []);
+
+  const promptInstall = async () => {
+    if (!event) return;
+    await event.prompt();
+    const { outcome } = await event.userChoice;
+    // The captured event is single-use; drop it either way so the button does
+    // not linger after a decision.
+    if (outcome === "accepted") setEvent(null);
+  };
+
+  return { canInstall: event !== null, promptInstall };
+}