[ACTV-451] remove 'intercom_desktop_enabled' feature flag

[augdiebold]

Related issues

• Closes ACTV-451

Proposed changes

Remove the intercom_desktop_enabled feature-flag gate from Anki desktop Intercom.

The launcher is no longer waiting on that remote flag. After this change it shows when the user is logged in and the Support button preference is on (ankihub_support_button, default true).

intercom_support_button_anki_preferences is unchanged: it still controls whether the Support button checkbox appears in AnkiHub preferences.

Tests that mocked or stored intercom_desktop_enabled were updated. Auth-app-url tests now use a generic cached flag dict so they are not tied to Intercom.

How to reproduce

1. Run Anki with this add-on build (staging or production).
2. Stay logged out on the deck browser: Intercom should not appear.
3. Log in to AnkiHub. Open the deck browser (home) or a deck overview: the Intercom launcher should appear without enabling intercom_desktop_enabled in PostHog/feature flags.
4. If the Support button checkbox is available (still behind intercom_support_button_anki_preferences), turn Support button off, close preferences, and return to home: Intercom should shut down.
5. Turn Support button back on: Intercom should boot again on home/overview.