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/ui_qt.py

@@ -1,4 +1,4 @@
-"""Qt progress UI for cloud-sync.
+"""Qt UI for instance-sync.
 
 Supports both PySide6 (preferred — LGPL, official Qt binding) and PyQt6
 (fallback — GPL/commercial). Same code runs on both because their
@@ -6,7 +6,7 @@ QtWidgets / QtCore APIs are interchangeable for our subset.
 
 This module never imports Qt at top level. ``import_qt()`` raises
 ImportError if neither binding is available; the factory in ``ui.py``
-catches that and falls back to the tkinter window.
+catches that and falls back to :class:`HeadlessProgress`.
 
 Threading model: ``QApplication`` runs on the main thread (started by
 ``run_with`` via ``QDialog.exec``); the restic worker runs on a daemon
@@ -35,7 +35,7 @@ def import_qt() -> tuple[Any, Any, Any]:
     except ImportError as e:
         raise ImportError(
             "neither PySide6 nor PyQt6 is installed; "
-            "pip install 'cloud-sync[qt]' or pip install PySide6"
+            "pip install PySide6 (or pip install 'cloud-sync[qt]')"
         ) from e
 
 
@@ -143,7 +143,7 @@ class QtProgressWindow:
             _apply_prism_dark(self._app)
 
         self._dialog = QtWidgets.QDialog()
-        self._dialog.setWindowTitle("Cloud sync")
+        self._dialog.setWindowTitle("Instance sync")
         self._dialog.setFixedSize(520, 240)
         self._dialog.setStyleSheet(_PROGRESS_QSS)
         self._dialog.setWindowFlag(
@@ -161,7 +161,7 @@ class QtProgressWindow:
         badge.setPixmap(icons.svg_pixmap(icons.SYNC_BADGE_SVG, 32))
         badge.setFixedSize(32, 32)
         header.addWidget(badge)
-        self._title_label = QtWidgets.QLabel("CLOUD SYNC")
+        self._title_label = QtWidgets.QLabel("INSTANCE SYNC")
         self._title_label.setObjectName("title")
         header.addWidget(self._title_label)
         header.addStretch(1)
@@ -326,7 +326,7 @@ def prompt_login_qt() -> str | None:
         _apply_prism_dark(app)
 
     dialog = QtWidgets.QDialog()
-    dialog.setWindowTitle("Cloud sync — connect account")
+    dialog.setWindowTitle("Instance sync — connect account")
     dialog.setFixedSize(560, 360)
     dialog.setStyleSheet(_LOGIN_QSS)
 
@@ -340,16 +340,16 @@ def prompt_login_qt() -> str | None:
     badge.setPixmap(icons.svg_pixmap(icons.PLUS_BADGE_SVG, 32))
     badge.setFixedSize(32, 32)
     header.addWidget(badge)
-    title = QtWidgets.QLabel("CONNECT CLOUD SAVE")
+    title = QtWidgets.QLabel("CONNECT TO THE NETWORK")
     title.setObjectName("title")
     header.addWidget(title)
     header.addStretch(1)
     outer.addLayout(header)
 
     body = QtWidgets.QLabel(
-        "To enable cross-machine save sync, message the Discord bot to "
-        "register this account. The bot will DM you a one-line token — "
-        "paste it below."
+        "To sync this instance across machines, register on the Timemachine "
+        "Network. Message the Discord bot — it will DM you a one-line token. "
+        "Paste it below."
     )
     body.setObjectName("body")
     body.setWordWrap(True)
@@ -380,7 +380,7 @@ def prompt_login_qt() -> str | None:
     outer.addStretch(1)
 
     foot = QtWidgets.QHBoxLayout()
-    skip = QtWidgets.QPushButton("Skip cloud sync")
+    skip = QtWidgets.QPushButton("Skip instance sync")
     skip.setObjectName("secondary")
     foot.addWidget(skip)
     foot.addStretch(1)
@@ -479,7 +479,7 @@ def prompt_conflict_qt(
 
     Args:
         local_modified: human-readable "Saturday, February 12 at 12:28 AM"
-        remote_modified: same, but for the cloud snapshot
+        remote_modified: same, but for the Timemachine Network snapshot
         save_label: noun phrase for body copy (e.g. "Minecraft save").
 
     Returns one of: ``"keep_local"``, ``"use_remote"``, ``"cancel"``.
@@ -527,7 +527,7 @@ def prompt_conflict_qt(
             self.clicked.emit()
 
     dialog = QtWidgets.QDialog()
-    dialog.setWindowTitle("Cloud sync — conflict")
+    dialog.setWindowTitle("Instance sync — conflict")
     dialog.setFixedSize(640, 460)
     dialog.setStyleSheet(_CONFLICT_QSS)
 
@@ -541,16 +541,17 @@ def prompt_conflict_qt(
     warning.setPixmap(icons.svg_pixmap(icons.WARNING_BADGE_SVG, 32))
     warning.setFixedSize(32, 32)
     header.addWidget(warning)
-    title = QtWidgets.QLabel("CLOUD CONFLICT")
+    title = QtWidgets.QLabel("INSTANCE CONFLICT")
     title.setObjectName("title")
     header.addWidget(title)
     header.addStretch(1)
     outer.addLayout(header)
 
     body = QtWidgets.QLabel(
-        f"Your local {save_label} conflicts with what is stored in the cloud. "
-        f"Whichever save data you choose to keep will be synced to this device "
-        f"and the cloud. The option you choose not to keep will be overwritten."
+        f"Your local {save_label} conflicts with what is stored on the "
+        f"Timemachine Network. Whichever save data you choose to keep will "
+        f"be synced to this device and the network. The option you choose "
+        f"not to keep will be overwritten."
     )
     body.setObjectName("body")
     body.setWordWrap(True)
@@ -560,7 +561,7 @@ def prompt_conflict_qt(
 
     cloud_card = _Card(
         icons.svg_pixmap(icons.CLOUD_SVG, 32),
-        "Cloud Save",
+        "Remote Save",
         f"Modified {remote_modified}",
     )
     local_card = _Card(