Let the user choose which settings persist

[marcinz606]

What

Which settings carry onto a freshly-opened file is now a user choice, edited in a Persistent Settings… picker on the canvas ⋯ menu.

Why

_apply_sticky_settings hard-coded the list across ~25 inline blocks reading 38 last_* global keys. The set had drifted: 11 of those keys had no reader at all (last_density, last_grade, last_wb_*, last_toe*, last_shoulder*, last_toning_config, last_retouch_config), 6 that were read got written from five other places, and nothing enumerated them.

How

settings_catalog.py already describes every copyable setting as a SettingRow and already knows how to overlay a chosen subset (apply_selected_fields). Sticky is the same operation with a different source, so it is rewritten on top of it rather than gated key by key:

• Store: one snapshot of the last persisted edit + one list of chosen row ids, both in the existing global_settings table.
• Apply: apply_selected_fields(snapshot, config, chosen_rows).
• Rows carry a sticky=True default flag, so out-of-the-box behaviour is unchanged (50 of 140 rows).

Both functions shrink to a few lines, and all 140 catalog rows become opt-in stickyable — a look setting can be opted in, a default one opted out.

Four carries stay hard-coded

They are not plain config-value copies:

Carry	Why
Flat-field profile_id/k1	rig-global, always overrides
Three wb_temp_lock* Kelvin locks	re-aim M/Y via kelvin_to_wb
21 ExportConfig fields	no catalog row — output folder, ICC paths, contact-sheet layout
linear_raw / narrowband_scan	kept out of the catalog on purpose: as Process rows they'd land in _LOOK_ROWS, so preset "Replace look" would reset the decode mode

description_fields keeps its own key so the last Description… confirm wins for the roll rather than whichever frame was saved last.

A EXPORT_REMAINDER test asserts the catalog's Export rows plus the remainder cover ExportConfig exactly — that's the guard stopping a future export field from silently losing its carry (the output folder nearly did).

UI

GranularSettingsDialog gains a preselect_ids pick mode (values shown come from the snapshot, so the list reads as what would actually carry), and CollapsibleSection gains an optional tri-state header checkbox. Both default off, so paste/preset/apply callers are untouched.

Migration

Legacy last_* keys seed the snapshot once on startup, including the pre-rename last_true_black → inverted paper_black.

Testing

• make lint, make type clean; 4574 passed.
• New: tests/test_sticky_settings.py (18), tests/test_sticky_dialog.py (6); tests/test_desktop_session.py rewritten onto the new storage plus 5 new behaviour tests.
• Driven end-to-end against the real app under Xvfb: menu item present, picker opens, choice persists, a ticked row carries onto a fresh file, an unticked one stops, and an already-edited file keeps its own look.

Note

tests/test_scanner_icc.py fails on this branch, but it fails identically on main — it is untracked WIP from another work stream and is not included here.