feat: add Playwright agent testability infrastructure

Add test harness (window.__bitvidTest__), data-testid attributes, relay
override support, and Playwright fixtures so agents can programmatically
login, seed relay data, and inspect app state during E2E testing.

- js/testHarness.js: exposes loginWithNsec, getAppState, relay overrides
- data-testid attrs on login/upload/video modals, header buttons, feed
- simple-relay.mjs: HTTP seeding API (POST /seed, GET /events, /health)
- bitvidTestFixture.ts: reusable Playwright fixture with mock relay
- agent-testability.spec.ts: E2E tests validating the infrastructure

https://claude.ai/code/session_01XnrszNmdqrAGd8g8qhAn4G

index.html

@@ -207,6 +207,7 @@
               name="query"
               placeholder="Search videos"
               autocomplete="off"
+              data-testid="search-input"
             />
             <button type="submit" class="header-search__submit">
               <span class="sr-only">Submit search</span>
@@ -264,6 +265,7 @@
               id="loginButton"
               class="btn focus-ring h-12 w-12 rounded-full p-0"
               aria-label="Open login modal"
+              data-testid="login-button"
             >
               <svg
                 class="h-9 w-9"
@@ -284,6 +286,7 @@
             <button
               id="uploadButton"
               class="btn focus-ring hidden h-12 w-12 rounded-full p-0 text-xl font-bold leading-none"
+              data-testid="upload-button"
             >
               +
             </button>
@@ -292,6 +295,7 @@
             <button
               id="profileButton"
               class="btn-ghost focus-ring relative hidden h-12 w-12 rounded-full p-0"
+              data-testid="profile-button"
             >
               <div class="h-10 w-10 overflow-hidden rounded-full">
                 <img