feat(ui): Qt progress window with Prism-Launcher-inspired dark palette

cloud-sync now ships a real Qt UI alongside the tkinter fallback.

Architecture:
  - HeadlessProgress: --no-gui path, plain stdout
  - TkProgressWindow: stdlib fallback when Qt isn't installed
  - QtProgressWindow: preferred path; supports both PySide6 and PyQt6
    (interchangeable APIs for our subset)

The factory in ui.py picks Qt → tkinter → headless. Tk stays so the
zipapp still works on bare Python with no extras.

Threading: QApplication runs on the main thread (started by run_with
via QDialog.exec). The restic worker runs on a daemon threading.Thread.
Cross-thread UI updates go via a Signal on a bridge QObject so Qt
auto-marshals them onto the main thread via a queued connection.

Cancellation: WM close + Cancel button both set a flag. sync.pull/push
pass ui.is_cancelled as restic.run's cancel_check; the subprocess gets
killed and returns -1 → exit 1.

Theme: Fusion style + Prism's dark palette (RGB values copied as facts
from PrismLauncher's DarkTheme.cpp). Override with PRISM_THEME=off.

Pyz size went 20 KB → 36 KB (added ui.py + ui_qt.py).
33 tests still green.

cloud_sync/sync.py

@@ -23,25 +23,33 @@ from pathlib import Path
 from . import restic, scope as scopemod
 from .cli import Args
 from .creds import read_credentials
+from .ui import HeadlessProgress, Progress
 
 
-def pull(args: Args) -> int:
+def pull(args: Args, progress: Progress | None = None) -> int:
     """Restore latest snapshot's files into pack_folder.
 
     If the repo has no snapshots yet, this is a no-op (first run on this
     machine; nothing to restore).
     """
+    ui = progress or HeadlessProgress()
+    ui.set_status("Reading credentials…")
     discord_id, password = read_credentials(args.token_file)
+
+    ui.set_status("Resolving restic binary…")
     binary = restic.resolve_binary(args)
     repo = _restic_repo(args.url, discord_id, password)
     env = _restic_env()
 
-    # Check whether any snapshots exist
+    ui.set_status("Checking remote snapshots…")
     code, out = restic.run(
         binary,
         ["-r", repo, "--insecure-no-password", "snapshots", "--json", "--latest", "1"],
         env=env,
+        cancel_check=ui.is_cancelled,
     )
+    if code == -1:
+        return 1
     if code != 0:
         print(
             f"cloud-sync: failed to list snapshots (restic exit {code})",
@@ -50,6 +58,7 @@ def pull(args: Args) -> int:
         return 2
     stripped = out.strip()
     if stripped in ("", "null", "[]"):
+        ui.set_status("No snapshots yet — nothing to pull")
         print(
             "cloud-sync: no snapshots yet for this user "
             "(first run on this machine?); nothing to pull"
@@ -59,6 +68,7 @@ def pull(args: Args) -> int:
     scope = scopemod.load(args.pack_folder)
     _, exclude_from = scopemod.materialize_for_restic(args.pack_folder, scope)
 
+    ui.set_status("Restoring files…")
     code, _ = restic.run(
         binary,
         [
@@ -68,17 +78,25 @@ def pull(args: Args) -> int:
             "--exclude-file", str(exclude_from),
         ],
         env=env,
+        cancel_check=ui.is_cancelled,
     )
+    if code == -1:
+        return 1
     if code != 0:
         print(f"cloud-sync: restic restore failed (exit {code})", file=sys.stderr)
         return 2
+    ui.set_status("Pull complete")
     print("cloud-sync: pull ok")
     return 0
 
 
-def push(args: Args) -> int:
+def push(args: Args, progress: Progress | None = None) -> int:
     """Snapshot the in-scope files into the user's repo."""
+    ui = progress or HeadlessProgress()
+    ui.set_status("Reading credentials…")
     discord_id, password = read_credentials(args.token_file)
+
+    ui.set_status("Resolving restic binary…")
     binary = restic.resolve_binary(args)
     repo = _restic_repo(args.url, discord_id, password)
     env = _restic_env()
@@ -86,6 +104,7 @@ def push(args: Args) -> int:
     scope = scopemod.load(args.pack_folder)
     files_from, exclude_from = scopemod.materialize_for_restic(args.pack_folder, scope)
 
+    ui.set_status("Uploading snapshot…")
     code, _ = restic.run(
         binary,
         [
@@ -98,10 +117,14 @@ def push(args: Args) -> int:
         ],
         env=env,
         cwd=args.pack_folder,
+        cancel_check=ui.is_cancelled,
     )
+    if code == -1:
+        return 1
     if code != 0:
         print(f"cloud-sync: restic backup failed (exit {code})", file=sys.stderr)
         return 2
+    ui.set_status("Push complete")
     print("cloud-sync: push ok")
     return 0