refactor(import): run every import source through one flow

[fullynocturnal]

Nine import sources each had their own screen and view model, and the flow behind them was the same in all nine: pick an input, preview it, choose what to bring across, submit, poll the job, report. They now share one. About 4,400 lines out, 1,300 in.

How it fits together

• ImportSources.kt has an ImportSource entry per source holding everything that differs: what the user supplies (a file, an encrypted file and its passphrase, or a token), how the preview is fetched and read, the fixed options fields, and the few real quirks, such as PluralKit's live pull going to the credential endpoint and a Sheaf complete-backup zip submitting under its own source.
• ImportViewModel and ImportScreen run the flow for whichever source the route names (settings/import/run/{source}).
• ImportSourcePickerScreen replaces the nine rows in Settings > Data with one "Import data" row and a list to choose from.

Adding a source is now an entry in the registry rather than two new files and a new route. Onboarding's Simply Plural shortcut goes through the same flow.

One options builder

The nine hand-written request builders become buildOptionsJson. The tests pin what each of them had to get right on its own:

• a category the export did not populate is still sent at its default, because the backend rejects unknown fields but expects the known ones;
• the sources that send an explicit "member_ids": null (Prism, PluralSpace, Ampersand) still do, and the others still omit it;
• a narrowed selection sends its ids;
• fixed fields such as conflict_strategy go out alongside the toggles.

Checked

• ImportBodyTest, extended with the builder cases above, and the full unit suite on this branch.
• assembleOpenDebug and assemblePlayDebug.
• Touches SheafApp.kt and SettingsCategoryScreens.kt like feat(sharing): manage public profiles and share links from the phone #78 and feat(settings): fold the Safety category into System #79 do, in different places; it merges cleanly with each of them, in either order.