feat(snippets): add snippets entity, crud, and auto-browser-per-run

- add Snippet entity with name, description, code; full CRUD backend
- add snippets pages (list, create, edit, detail) and nav entry
- add runSnippet helper in code-executor using new Function with args array
- add result param to execute() so validateCode can access exec output
- remove sessionName from steps; each run now spawns its own fresh browser
- fix waitForURL race by polling localStorage for token instead

server/src/snippet/snippet.entity.ts

@@ -0,0 +1,34 @@
+import {
+  Entity,
+  PrimaryGeneratedColumn,
+  Column,
+  CreateDateColumn,
+  UpdateDateColumn,
+} from "typeorm";
+
+@Entity("snippets")
+export class SnippetEntity {
+  @PrimaryGeneratedColumn()
+  id: number;
+
+  /** Unique identifier used to call the snippet: helpers.runSnippet('mySnippet', ...) */
+  @Column({ unique: true })
+  name: string;
+
+  @Column("text", { nullable: true })
+  description: string | null;
+
+  /**
+   * The snippet body — written as a regular async function body.
+   * It receives the same `page`, `context`, and `helpers` as exec code, plus
+   * any positional `...args` passed by the caller.
+   */
+  @Column("text")
+  code: string;
+
+  @CreateDateColumn()
+  createdAt: Date;
+
+  @UpdateDateColumn()
+  updatedAt: Date;
+}