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/api/spa_test.go

@@ -77,6 +77,16 @@ func TestSPAMissingAssetIs404(t *testing.T) {
 	}
 }
 
+func TestSPADirectoryIs404NotListing(t *testing.T) {
+	// A request that resolves to an embedded directory must 404, never a listing.
+	for _, p := range []string{"/assets", "/assets/"} {
+		w := do(t, testEngine(), http.MethodGet, p)
+		if w.Code != http.StatusNotFound {
+			t.Errorf("GET %s status = %d, want 404", p, w.Code)
+		}
+	}
+}
+
 func TestSPAUnmatchedAPIIs404JSON(t *testing.T) {
 	w := do(t, testEngine(), http.MethodGet, "/api/v1/nope")
 	if w.Code != http.StatusNotFound {