rename: cloud sync -> instance sync; cloud -> Timemachine Network; drop Tk

Product / UI / CLI / docs rebrand. Internal package, repo, and
on-disk dir names stay 'cloud_sync' / 'cloud-sync' / '.cloud-sync/'
to avoid breaking existing installs; a future commit can do the
file-system rename when the cost is worth paying.

User-facing changes:
  CLI prog name:        cloud-sync     -> instance-sync
  CLI description:      cloud-svc URL  -> Timemachine Network endpoint
  Dialog title:         CLOUD SYNC     -> INSTANCE SYNC
  Dialog title:         CLOUD CONFLICT -> INSTANCE CONFLICT
  Dialog title:         CONNECT CLOUD SAVE -> CONNECT TO THE NETWORK
  Card label:           Cloud Save     -> Remote Save
  Skip button:          Skip cloud sync -> Skip instance sync
  Body copy:            'the cloud'    -> 'the Timemachine Network'
  Window titles:        Cloud sync — ... -> Instance sync — ...
  Log prefix:           cloud-sync:    -> instance-sync:
  Error prose:          'cloud-sync token' -> 'instance-sync token'

Backend changes:
  restic --host tag:    cloud-sync     -> instance-sync
  State.host_tag dflt:  cloud-sync     -> instance-sync
  (Existing snapshots with the old tag still pull fine; we use 'latest'.)

Drop tkinter fallback: ui.py now offers Qt OR Headless. tkinter is
unnecessary given we already maintain Qt + headless; one less code
path to keep styled, smaller pyz. make_progress() picks Qt first,
falls through to HeadlessProgress on ImportError with a stderr hint
to 'pip install PySide6'.

README: rebrand title + prose; note repo/dir rename deferred; call
out the PySide6 install step. Conflict/login dialogs are now Qt-only;
without Qt, conflict aborts (defensive) and login tells the user to
paste the token manually.

52 tests green; no test-file label changes needed since they only
exercise internal APIs.

cloud_sync/restic.py

@@ -8,7 +8,7 @@ Discovery order:
 
 The version is pinned because repos written by one restic version can have
 features another version can't read. Cache the pinned binary per-instance
-so deleting the instance dir wipes everything cloud-sync owns.
+so deleting the instance dir wipes everything instance-sync owns.
 """
 
 from __future__ import annotations
@@ -203,7 +203,7 @@ def _download_restic_to(target: Path, plat: Platform) -> None:
         f"{RELEASE_TAG}/SHA256SUMS"
     )
     print(
-        f"cloud-sync: downloading restic {RESTIC_VERSION} from {asset_url}",
+        f"instance-sync: downloading restic {RESTIC_VERSION} from {asset_url}",
         file=sys.stderr,
     )
     with tempfile.NamedTemporaryFile(suffix=f".{ext}", delete=False) as tmp: