feat(sync): built-in Nextcloud sync (Issue #49, stage 2)

Sign in to any Nextcloud server (also private ones in a subfolder, with
their own port or a self-signed certificate) through the browser with
Login Flow v2; the app gets an app password of its own, which
"Disconnect" revokes again.

A background thread syncs the storage folder with the folder ink-notes
on the server every 30 seconds and shortly after saving, three-way
against the state after the last pass: one-sided changes are copied,
manifests and library.json are merged, a page changed on both sides is
kept twice (the server's version as conflict copy, which disk_watch
turns into a page), empty pages give way, deletions go into the trash
on both sides and lose against changes. Changes to the open notebook
wait while it has unsaved changes. Folder ETags spare listing unchanged
notebooks; the pass stops instead of deleting when the storage folder
or the server folder is missing.

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

Cargo.lock

@@ -1565,6 +1565,7 @@ dependencies = [
  "pollster",
  "quick-xml",
  "resvg",
+ "rustls",
  "semver",
  "serde",
  "serde_json",
@@ -1576,6 +1577,7 @@ dependencies = [
  "wayland-client",
  "wayland-protocols",
  "webbrowser",
+ "webpki-roots 0.26.11",
  "wgpu",
  "winit",
 ]