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/api/settings_test.go

@@ -1,7 +1,9 @@
 package api
 
 import (
+	"encoding/json"
 	"net/http"
+	"strings"
 	"testing"
 
 	"github.com/gin-gonic/gin"
@@ -159,10 +161,22 @@ func TestSettingsRejectsBadModel(t *testing.T) {
 	admin := registerAndCookie(t, r, "[email protected]")
 	v := settingsVersion(t, r, admin)
 
-	if w := doJSON(t, r, http.MethodPatch, "/api/v1/settings",
-		map[string]any{"agentModel": "nonesuch/model", "version": v}, admin); w.Code != http.StatusBadRequest {
+	w := doJSON(t, r, http.MethodPatch, "/api/v1/settings",
+		map[string]any{"agentModel": "nonesuch/model", "version": v}, admin)
+	if w.Code != http.StatusBadRequest {
 		t.Errorf("bad model: status %d, want 400", w.Code)
 	}
+	// The message says which field and which spec, so the page can show a reason
+	// rather than a bare "invalid input".
+	var body struct {
+		Error struct{ Code, Message string } `json:"error"`
+	}
+	if err := json.Unmarshal(w.Body.Bytes(), &body); err != nil {
+		t.Fatalf("decode bad-model body: %v", err)
+	}
+	if body.Error.Code != "INVALID_INPUT" || !strings.Contains(body.Error.Message, "chat model") || !strings.Contains(body.Error.Message, "nonesuch/model") {
+		t.Errorf("bad model error = %+v, want INVALID_INPUT naming the chat model and spec", body.Error)
+	}
 	// agentEnabled must be a bool or null, not a string.
 	if w := doJSON(t, r, http.MethodPatch, "/api/v1/settings",
 		map[string]any{"agentModel": "", "agentEnabled": "yes", "version": v}, admin); w.Code != http.StatusBadRequest {