Home page: auto-connect last-used database + launch buttons for sessions

[peterhollender]

Follow-up to SlicerOpenLIFU#634 (Session Overview simplification) and part of the split-session refactor tracked by SlicerOpenLIFU#631 / SESSION_SPLIT_DESIGN.md.

Problem

The v2 Home page currently shows read-only status labels and a single "Open Data Manager" button. Two regressions vs. the legacy app:

1. No auto-connect to the last-used database. In the legacy app, OpenLIFU/databaseDirectory was persisted in QSettings when a database loaded, and read back at Slicer startup so users didn't have to re-open the Database dialog every launch. The v2 Data Manager writes nothing to QSettings and reads nothing on startup.
2. Data Manager is being used as the "launching point" for sessions, but that's not its job. Data Manager is the admin panel for managing subjects / planning sessions / plans / sonication sessions / solutions / protocols / transducers / users. It's fine for it to also be able to jump into a session, but the primary entry point for a clinical workflow should be big, obvious buttons on the landing page.

Proposed fix

A. Auto-connect

• When DatabaseLogic.load_database(path) succeeds, persist path to QSettings("OpenLIFU/databaseDirectory").
• When DatabaseLogic.unload_database() runs, clear the setting so the user's explicit disconnect sticks across restarts.
• Add DatabaseLogic.try_auto_connect() -> bool that reads the setting, validates it with sample_data.path_is_openlifu_database_root(path), and calls load_database(path). Returns True on success. All errors swallowed + logged.
• OpenLIFUHomeWidget.enter() calls try_auto_connect() before refresh_status(), but only if no DB is currently loaded.
• Auto-connect is a Home-only trigger. No other page should invoke it. Database connect/disconnect stays user-driven from the Data Manager.

Design decision: the auto-connect is a single-shot on landing on Home, not a Slicer-startup hook. That keeps the connect/disconnect model app-wide (single source of truth: DatabaseLogic.db) without adding startup-timing hooks, and matches user expectation ("I opened OpenLIFU → my last database is there").

B. Launch buttons on Home

Replace the current single "Open Data Manager" button with a set of prominent action buttons:

• New Planning Session — opens a New PlanningSession dialog (the same one Data Manager uses), writes to the database, loads the new session into app state, navigates to OpenLIFUPlanningSessionOverview.
• Continue Planning Session — opens a picker (subject combo + planning session list), loads the chosen session, navigates to overview.
• New Sonication Session — opens a New SonicationSession dialog (requires an existing Plan for the chosen subject), writes to the database, loads, navigates to OpenLIFUSonicationSessionOverview.
• Continue Sonication Session — opens a picker (subject combo + sonication session list), loads the chosen session, navigates to overview.

Plus a smaller "Open Data Manager" link at the bottom for admin access.

Scope hygiene

• Shared session-lifecycle actions currently baked into OpenLIFUDataManagerLogic (create_planning_session, create_sonication_session, load_planning_session, load_sonication_session, close_loaded_sessions) get pulled into OpenLIFUApp/logic/session_actions.py module-level functions. DataManagerLogic delegates. Home calls the module functions directly. This keeps Rule 6 (no cross-page reach-ins into another page's logic).
• Shared dialogs NewPlanningSessionDialog / NewSonicationSessionDialog get moved from data_manager_page.py into OpenLIFUApp/dialogs/session_dialogs.py (Rule 7 — any dialog used from >1 page moves to dialogs/). New ContinuePlanningSessionDialog / ContinueSonicationSessionDialog live in the same file.

Non-goals

• No sign-in / hardware-connect / cloud-sync gating on Home. (Separate features, later.)
• No timeline navigation on Home — the timeline is a within-workflow footer, not a landing-page element.
• No auto-connect anywhere except Home.

Acceptance

1. Launch Slicer, load a database from the Data Manager, exit. Re-launch. Home shows the database as already-connected, labels populated.
2. Click Disconnect (via Data Manager). Exit. Re-launch. Home shows no database and the "New / Continue" buttons show a "Load a database first" hint.
3. From Home, click "New Planning Session" → dialog opens → fill out → OK → Planning Session Overview page appears with the new session loaded.
4. Similar for the other three buttons.
5. Data Manager still works from its own "Load…" button; auto-connect does not fire while navigating between pages.

Relates to SlicerOpenLIFU#631, SlicerOpenLIFU#634.