[gedaiu]

The web form for RC-003 (a source may decay). Until now the option could only be set with PATCH (#19); the source settings dialog now has it.

What is in it.

• A "Let documents decay" switch in a source's settings, for every source kind. On, it shows the four numbers (keep at most, score halves every, rank lower after, never delete newer than) with the backend's defaults and a hint saying what happens: the lowest-scoring old documents are deleted once a day and can be restored.
• Save sends decay only when it changed: the whole object when switched on or edited, null when switched off, nothing otherwise. A hand-written {} is read with the defaults and is not rewritten by an unrelated save.
• A number out of the backend's range (decayIssue, same rules as DecaySettings::validate) marks its label and keeps Save disabled.
• SourceView.decay and DecaySettings in api.ts; English and Chinese strings; one line in docs/design/sources.md.
• The logic is in a pure sourceDecay.ts with its test, the fields in SourceDecayFields.tsx, and Library.tsx only wires it in.

Tests. 16 for the helper (defaults, range, what a save sends), 3 rendering the fields (off shows only the switch, on shows four inputs with saved values, a bad field's label is marked). Web: tsc --noEmit, vitest run (334 passed), the style guard and vite build pass.

Not verified. I have not looked at it in a browser against a running server; please click through the dialog once. Create does not take decay (set it after creating).

🤖 Generated with Claude Code