Smoke-sweep fixes: exact saves, local dates, safer remove, readable markers

- Garden and plant dialogs keep centimeters as the source of truth
  (LengthField in lib/units.ts): a no-change Save no longer rewrites
  900 cm as 899.922 or a 45 cm spacing as 44.958, bumping versions and
  writing bogus history entries on the way.
- The UI stamps every date with the browser's local day (lib/dates.ts).
  Journal notes already did; plop placement, fill and removal now do too,
  so a 9 pm placement isn't "planted tomorrow". The fill endpoint gained an
  optional plantedAt; API and agent callers still default to UTC today.
- Removing an object that holds plants asks first and says how many go
  with it. An empty one still goes straight away (one Undo restores it).
- The expanded plant card's action row wraps instead of clipping "Delete".
- Monogram lettering switches to a dark ink on pale marker colors (garlic,
  cabbage, marigold) instead of near-white on near-white.
- Copy-as-plan proposes the next free year and warns when the typed name
  already exists, so two gardens can't both read as "the 2027 plan".
- Plan cards show the base name with a "2027 plan" tag, so the year — the
  point of the name — survives truncation.
- A rejected model spec now says which model and why: a wrapped
  ErrInvalidInput's reason reaches the client as the 400's message, and the
  Settings field shows it inline instead of toasting "invalid input".

Also defuses a clock bomb in TestRemainingReturnsWhenAPlantingIsRemoved,
which only passed while the real date was before 2026-08-01.

Co-Authored-By: Claude Fable 5 <[email protected]>

CLAUDE.md

@@ -129,6 +129,21 @@ Conventions that follow from it:
   a constraint between neighbouring plants; a bed edge is nobody's neighbour.
 - **Soft removal**: "clear bed" sets `removed_at`; the editor reads
   `removed_at IS NULL`. Hard delete is a different operation.
+- **Length fields keep centimeters as the source of truth.** A dialog field
+  that takes a length is a `LengthField` (`web/src/lib/units.ts`): the text is
+  a view, `cm` changes only when the person types. Never re-parse the display
+  string on save — "29′ 6.3″" is the nearest tenth of an inch, and parsing it
+  back is how a no-change Save turned 900 cm into 899.922 (and bumped the
+  version, and wrote a bogus history entry). The inspector guards the same way.
+- **"Today" is the browser's local day**, from `today()` in
+  `web/src/lib/dates.ts`, and the UI always sends it: journal `observedAt`,
+  plop/fill `plantedAt`, `removedAt`. The server's UTC default is only for
+  API callers and the agent. A gardener placing at 9 pm in Ohio planted today,
+  not tomorrow — don't add a UI path that leaves the date to the server.
+- **A wrapped `ErrInvalidInput` is shown to the person verbatim.**
+  `fmt.Errorf("%w: chat model %q: unknown provider", domain.ErrInvalidInput, spec)`
+  reaches the client as the 400's message (minus the sentinel prefix); the bare
+  sentinel reads "invalid input". Write the reason for the keyboard, not the log.
 - **Migrations** are numbered `.sql` files in `internal/store/migrations/`, run
   at startup, embedded. Never edit one that has shipped.
 - **Every service mutation lands in history** (#48). If you add one, record it —