Seed-packet scan UI: camera/upload → proposal → confirm (#102)

The seed-packet capture backend has been live since #94 (POST /seed-lots/scan
and /seed-lots/from-packet, with /capabilities reporting `vision`), but there was
no UI, so the feature Steve asked for — "take a picture of a packet and it
auto-creates" — was unreachable. This adds the mobile-first UI.

- Entry point in the Plants catalog ("Scan a packet"), shown only where
  `capabilities.vision` is on — so it's never a dead button on an instance with
  no vision model. Adds `vision` to the capabilities schema (both flags now
  default false, so a partial response hides a feature rather than failing parse).
- ScanPacketModal: two phases in one dialog. Capture uses a file input with
  `accept="image/*" capture="environment"` so a phone opens the camera directly
  (also picks an existing photo; HEIC is handled server-side). Review shows the
  extracted fields read-only for context, then an editable confirm — pick a ranked
  existing candidate (with its match reason) or create a new variety prefilled from
  the packet. Nothing commits blind, which is the whole point of the backend's
  never-auto-create design.
- New data layer (lib/seedPacket.ts): zod schemas mirroring the backend
  (SeedPacket / PacketProposal / PacketResult), the scan + confirm mutations, and
  the pure prefill helpers (newPlantDefaults / lotDefaults) — unit-tested, since
  the prefill mapping (unknown category, missing spacing, seed-count→unit) is
  where the bugs hide.
- api.ts learns to send FormData as multipart (no JSON content-type) for the
  scan upload, via a new api.postForm.

Frontend only; no backend or route change. Last open child of epic #96.

Live camera→vision→proposal verification needs a vision-configured instance
(the flow is model-gated and unreachable locally, same as the assistant);
the confirm half and all prefill logic are covered by tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/lib/seedLots.ts

@@ -46,7 +46,7 @@ export const seedLotSchema = z.object({
 })
 export type SeedLot = z.infer<typeof seedLotSchema>
 
-const seedLotsKey = ['seed-lots'] as const
+export const seedLotsKey = ['seed-lots'] as const
 
 export function useSeedLots() {
   return useQuery({