Address Gadfly review findings on Phase 0

Applied the fixes warranted by the adversarial review of PR #21 (all
findings graded in the gadfly store):

Store (highest impact):
- buildDSN always merges busy_timeout/journal_mode/foreign_keys pragmas
  into the DSN, even for file: URIs or paths with a query string — the
  prior passthrough silently disabled FK enforcement for those PANSY_DB
  values. Also escape '#' in the path and tighten in-memory detection to
  an exact ':memory:' token or mode=memory param (no substring misfire).
- migrate.go: detect duplicate migration versions with a clear error;
  wrap appliedVersions' rows.Err() with the store: prefix.

API:
- SetTrustedProxies failure now falls back to trust-none instead of
  leaving gin's trust-everyone default (X-Forwarded-For spoofing).
- SPA: 404 embedded directories (was a directory listing), 404 bare /api,
  add X-Content-Type-Options: nosniff, cache index.html bytes once at
  startup, single fs.Stat existence check, shared writeAPIError helper.
- Move gin.SetMode out of package init() into New().

Config: validate PANSY_PORT range (fall back to 8080 with a warning).

Web:
- api.ts: serialize the request body before the fetch try so a
  JSON.stringify failure isn't misreported as a network error.
- AppShell: move state-specific/conflicting utilities into
  active/inactiveProps so concatenated Tailwind classes don't collide.

Tests: added store DSN-pragma/memory-detection cases and SPA
directory-404 case. go build/vet/test clean (CGO off); web tsc + build
clean; re-verified in a browser (SPA, deep links, /assets 404, nav).

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

internal/store/migrate.go

@@ -69,7 +69,10 @@ func (d *DB) appliedVersions(ctx context.Context) (map[int]bool, error) {
 		}
 		applied[v] = true
 	}
-	return applied, rows.Err()
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("store: iterate schema_migrations: %w", err)
+	}
+	return applied, nil
 }
 
 func (d *DB) applyMigration(ctx context.Context, m migration) error {
@@ -99,6 +102,7 @@ func loadMigrations() ([]migration, error) {
 	}
 
 	var migrations []migration
+	seen := map[int]string{}
 	for _, e := range entries {
 		if e.IsDir() || !strings.HasSuffix(e.Name(), ".sql") {
 			continue
@@ -107,6 +111,10 @@ func loadMigrations() ([]migration, error) {
 		if err != nil {
 			return nil, err
 		}
+		if prev, dup := seen[version]; dup {
+			return nil, fmt.Errorf("store: duplicate migration version %d: %q and %q", version, prev, e.Name())
+		}
+		seen[version] = e.Name()
 		body, err := fs.ReadFile(migrationsFS, "migrations/"+e.Name())
 		if err != nil {
 			return nil, fmt.Errorf("store: read migration %s: %w", e.Name(), err)