Address Gadfly review on #7: garden cap, dim validation, shared errors

Fixes from the PR #26 adversarial review (graded 18 real / 0 false positive).

Correctness / security
- maxGardenCM fixed to 10_000 (100 m), matching its comment — it was
  100_000 cm (1 km), 10x too lax (5 models flagged this).
- Dimension validation now rejects NaN/Inf (which slip past naive
  comparisons) and subnormal-tiny positives, via a finite [1cm, 100m]
  check. Name (200) and notes (10_000) are length-capped so untrusted
  input can't balloon storage.
- Update version binding is `required,min=1`, so a negative/zero version
  is a 400, not a 409.

Maintainability / performance
- One unified writeServiceError (new errors.go) maps every auth + resource
  sentinel; writeResourceError removed. writeVersionConflict and
  parseIDParam moved to errors.go (shared, not in the gardens feature file).
- Request structs share an embedded gardenFields (one toInput).
- CreateGarden uses INSERT ... RETURNING (one round-trip).
- ListGardensForOwner has a defensive LIMIT (pagination is post-v1).

Tests: name/notes length, NaN/Inf/subnormal dims, dimension-at-cap valid,
negative/zero version -> 400.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

internal/service/gardens_test.go

@@ -3,6 +3,8 @@ package service
 import (
 	"context"
 	"errors"
+	"math"
+	"strings"
 	"testing"
 
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
@@ -45,17 +47,27 @@ func TestCreateGardenValidation(t *testing.T) {
 	owner := seedUser(t, s, "[email protected]")
 
 	cases := []GardenInput{
-		{Name: "   "},                           // blank name
-		{Name: "X", UnitPref: "furlongs"},       // bad unit
-		{Name: "X", WidthCM: -5},                // non-positive dim (defaults only fill 0)
-		{Name: "X", WidthCM: maxGardenCM + 1},   // over the cap
-		{Name: "X", HeightCM: maxGardenCM * 10}, // over the cap
+		{Name: "   "}, // blank name
+		{Name: strings.Repeat("a", maxGardenNameLen+1)},              // name too long
+		{Name: "X", Notes: strings.Repeat("b", maxGardenNotesLen+1)}, // notes too long
+		{Name: "X", UnitPref: "furlongs"},                            // bad unit
+		{Name: "X", WidthCM: -5},                                     // negative (defaults only fill exact 0)
+		{Name: "X", WidthCM: maxGardenCM + 1},                        // over the cap
+		{Name: "X", HeightCM: maxGardenCM * 10},                      // over the cap
+		{Name: "X", WidthCM: math.NaN()},                             // NaN slips past naive < / >
+		{Name: "X", HeightCM: math.Inf(1)},                           // +Inf
+		{Name: "X", WidthCM: math.SmallestNonzeroFloat64},            // subnormal, > 0 but < 1 cm
 	}
 	for i, in := range cases {
 		if _, err := s.CreateGarden(context.Background(), owner, in); !errors.Is(err, domain.ErrInvalidInput) {
 			t.Errorf("case %d: err = %v, want ErrInvalidInput", i, err)
 		}
 	}
+
+	// A dimension exactly at the cap is valid.
+	if _, err := s.CreateGarden(context.Background(), owner, GardenInput{Name: "Big", WidthCM: maxGardenCM, HeightCM: maxGardenCM}); err != nil {
+		t.Errorf("dimension at cap should be valid: %v", err)
+	}
 }
 
 func TestListGardensOwnedOnly(t *testing.T) {