Clickable empty state, canvas-tracked centring, foldable sidebar branding

[linkmodo]

Summary

Three UI changes, two of them fixing real misbehaviour.

1. The empty-state prompt is now a button. "Load some scans to get started" opens a small menu with Add files… / Add folder…, so a first-run user can import without first finding the film-strip toolbar. FileBrowser._on_add_files/_on_add_folder become public prompt_add_files/prompt_add_folder; the overlay drives them through signals rather than reaching into the browser.

2. The empty state now tracks the canvas, not the window. This was a real bug: hiding a dock resizes the canvas without resizing the window, and the overlay's geometry was only re-applied from MainWindow.resizeEvent — so it kept its old size while the floating toolbar (which the canvas lays out itself) moved. Measured with both docks hidden:

before fix:  overlay 691x934  vs canvas 1484x934   → text centre 345 vs toolbar centre 741
after  fix:  overlay 1484x934 vs canvas 1484x934   → 741 vs 741

An event filter on the parent re-applies the canvas rect on every canvas resize. Verified aligned (±1px) across all four dock combinations.

3. The sidebar branding folds away. A chevron at the header's top-right collapses the logo + version, 70px → 15px, persisted as section_expanded_app_header using the same get_global_setting/expanded_changed pattern as the Analysis section, so it survives a restart.

An earlier revision of this PR also replaced the docked Session / Controls captions with a slim unnamed bar. That has been reverted at the author's request — the docks keep their native title bars, and pinnable_dock.py / test_dock_panels.py are untouched by this PR.

Test plan

• New tests/test_empty_state_and_header.py (11 tests): the prompt is a button, the menu offers both import routes, the tour signal fires, the overlay follows and stays centred on its parent through resizes, the header collapses/expands and frees height, and SessionPanel persists and restores the header state across rebuilds.
• All three changes sabotage-verified: reverting each one individually fails the tests that cover it.
• Driven against a real MainWindow: 25/25 checks pass, including the dock-toggle alignment matrix above and confirmation that both docks keep their native captions.
• Full suite: 3305 passed. The 4 failures (test_effective_input_icc, test_output_dir_subfolder_of_source, test_sidecar_path, test_watcher_skips_ir_sidecars) are pre-existing Windows path-separator issues, confirmed failing on a clean main.
• make format / ruff check clean; docs/USER_GUIDE.md updated for the clickable prompt and the collapsible header.