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/scope.py

@@ -1,8 +1,9 @@
 """Per-distribution sync scope (include/exclude paths).
 
-Each cloud-sync deployment ships its own ``scope.json`` that picks which
-files participate in sync. Lives at ``<pack-folder>/.cloud-sync/scope.json``.
-Defaults are baked in so a fresh install with no scope.json works.
+Each instance-sync deployment ships its own ``scope.json`` that picks
+which files participate in sync. Lives at
+``<pack-folder>/.cloud-sync/scope.json``. Defaults are baked in so a
+fresh install with no scope.json works.
 """
 
 from __future__ import annotations
@@ -48,7 +49,7 @@ def load(pack_folder: Path) -> Scope:
         data = json.loads(path.read_text(encoding="utf-8"))
     except (OSError, json.JSONDecodeError) as e:
         print(
-            f"cloud-sync: scope.json invalid ({e}); using defaults",
+            f"instance-sync: scope.json invalid ({e}); using defaults",
             file=sys.stderr,
         )
         return Scope()