feat(plantare): UI — propose from chat, review & sign on the ledger

The viral loop, made usable end-to-end:

- Propose sheet (plantare_propose_sheet.dart): from a chat with a peer
  (their key already in hand), pick your side, name the seed, choose what
  comes back (similar · non-GMO organic / work hours / other) and an
  optional return-by, then sign and send. Reached from the chat overflow
  menu; confirms "waiting for them to sign".
- Plantares screen: incoming proposals show Accept & sign / Decline right
  on the tile; every bilateral row wears a handshake badge — awaiting
  signature, signed by both, or declined. Accept falls back to a gentle
  "offline" nudge if the proposal isn't in hand yet.
- i18n: full en + es for the bilateral flow; other locales fall back to
  base per slang config.

Wire-up only touches human copy (no jargon) and stays behind the
optional PlantareService, so inventory-only builds are unaffected.

Tests: propose sheet drives a real sign+send; the ledger renders the
signed/awaiting badges. Fake SocialSession in your_people test gains the
new transport getter. Widget tests targeted + timed. All green.

apps/app_seeds/lib/i18n/strings.g.dart

@@ -4,9 +4,9 @@
 /// To regenerate, run: `dart run slang`
 ///
 /// Locales: 7
-/// Strings: 3396 (485 per locale)
+/// Strings: 3442 (491 per locale)
 ///
-/// Built on 2026-07-14 at 09:00 UTC
+/// Built on 2026-07-14 at 09:22 UTC
 
 // coverage:ignore-file
 // ignore_for_file: type=lint, unused_import