Add unified WeblinkSession controller and interaction-driven playback

Web links were implemented three times (main.py subprocess, run_win.py
subprocess + Win32 overlay, cef_browser.py embedded CEF), each owning its own
process handle, watchdog and teardown. That ambiguity caused leaked browsers,
skipped items, lost foreground and blank screens when a page failed to load.

Replace all three with a single owner in src/weblink_session.py:

- WeblinkSession: validate -> launch -> verify -> watch -> teardown.
  Generation-tokened so stale callbacks are ignored, idempotent close(),
  atexit-safe, never more than one browser alive.
- WeblinkAdapter: the only platform-specific part (launch / wait_visible /
  is_alive / teardown / prewarm). Platform layers inject engines through
  SignagePlayer.weblink_adapter_factory.
- ChromiumSubprocessAdapter: default engine (Pi chromium, Windows chrome/msedge).
- InteractionWatcher: decides when an item is finished.
- WebInputSources: /dev/input/event* (Linux) plus a GetCursorPos pointer tap
  (needed on Windows for embedded CEF, which has no child process).

Interaction model: web links are an interactive surface, not timed media.
The player advances only when the configured duration has elapsed AND the
viewer has not interacted for 10s, measured from the most recent interaction.
A touch in the final seconds of a slot therefore pushes the advance 10s past
that touch, and each further touch pushes it again, so a page is never pulled
out from under someone using it. An untouched page still advances on schedule.
A drag burst counts as one interaction but the countdown tracks its last event,
so an item cannot be cut off mid-gesture. max_dwell (duration x factor, floored
by min_max_dwell) is an absolute backstop against a wedged browser or a jammed
touchscreen.

Verified start-up: the visibility wait runs on the watcher thread, never on
Kivy's main thread. If the browser window never appears the item is reported
failed and skipped, instead of resetting the error counter and leaving a black
screen up for the whole duration.

Config: new "weblink" block in config/app_config.json (engine,
interaction_postpone, interaction_debounce, interaction_grace,
max_dwell_factor, min_max_dwell, launch_timeout, prewarm) with safe defaults,
so an absent block still works.

Also add weblink_session to the PyInstaller hiddenimports so the frozen exe
bundles the new module.

windows/build.spec

@@ -105,6 +105,11 @@ hidden_imports = [
     'cef_browser',
     'win32gui',
     'win32con',
+    # Unified web-link controller (launch / verified visibility / interaction
+    # watcher / teardown) — imported by main.py and run_win.py
+    'weblink_session',
+    # Windows-native card reader (Raw Input API + LL-hook fallback)
+    'win_card_reader',
 ]
 
 # Exclude Linux-only modules