fix: SPA routing and prompts table handling

- Add clean URL routing: /dashboard -> dashboard.html, /settings -> settings.html
- Add EnsurePromptsTable for dev environments where prompts table may not exist
- Fix nav links to use clean URLs instead of .html paths

Co-Authored-By: Claude <noreply@anthropic.com>

internal/db/db.go

@@ -109,6 +109,24 @@ func AutoMigrate() error {
 	return nil
 }
 
+// EnsurePromptsTable creates the prompts table if it doesn't exist (for dev environments)
+func EnsurePromptsTable() {
+	_, err := DB.Exec(`CREATE TABLE IF NOT EXISTS prompts (
+		id bigint NOT NULL AUTO_INCREMENT,
+		session_id varchar(128) NOT NULL,
+		project_name varchar(255) NOT NULL DEFAULT '',
+		prompt text NOT NULL,
+		created_at datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
+		PRIMARY KEY (id),
+		KEY idx_session_id (session_id),
+		KEY idx_project_name (project_name),
+		KEY idx_created_at (created_at)
+	) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`)
+	if err != nil {
+		log.Printf("Warning: failed to ensure prompts table: %v", err)
+	}
+}
+
 func SeedData() error {
 	// Check if system tags already exist
 	var count int