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]>

internal/domain/domain.go

@@ -33,7 +33,10 @@ var (
 	ErrShareExists = errors.New("garden already shared with that user")
 
 	// ErrInvalidInput means the caller supplied structurally invalid data (empty
-	// required field, malformed value). Mapped to 400.
+	// required field, malformed value). Mapped to 400. Wrap it with the reason —
+	// fmt.Errorf("%w: plantedAt must be a YYYY-MM-DD date", ErrInvalidInput) —
+	// and the API shows that reason to the person verbatim, so write it for
+	// them, not for a log; the bare sentinel reads as just "invalid input".
 	ErrInvalidInput = errors.New("invalid input")
 	// ErrInvalidCredentials means a login attempt failed. It is deliberately
 	// identical for an unknown email and a wrong password so neither can be