refactor(frontend): e2e tests setup + speed + readability (#10388)

## Changes 🏗️

### User creation tests

Now, all tests use the users created via the platform signup in
`global-setup.ts`. Their login details are on a `.auth/user-pool.json`
file. I have the delete the logic that created tests users via Supabase
directly.

### Build tests speed

I have refactored the builder tests, so that, instead of adding 100s of
blocks under a given test user session, a new test user logins and adds
block for each letter:
```
Test user 1
  - logins and adds blocks starting with "a"
Test user 2
  - logins and adds blocks starting with "b"
```
Given that we know the builder becomes slow once we have 30 or more
blocks, in this way a test user never adds more than 10 blocks on a
given test ( _without losing coverage_ ), so we don't need time-outs or
artificially waiting due to the UI being slow.

### Readability test changes

Refactor existing tests, using short-hand utilities, to be:
- easier to write
- clearer to read
- easier to debug

```ts
// Selectors
getId("id") // --> page.getByTestId("id")
getText("foo") // --> page.getByText("id")
getButton("Run") // --> page.getByRole("button", {name: "Run"}
...
// Assetions
const btn = getButton("Save")
isVisible(btn) // --> expect(btn).toBeVisible()
```
These utilities live under `selectors.ts` and `assertions.ts`. Their
usage is optional but encouraged.


## Checklist 📋

### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Refactored tests code looks good
  - [x] E2E tests are 🟢 on the CI 

### For configuration changes:

No config changes

.github/workflows/platform-frontend-ci.yml

@@ -155,8 +155,6 @@ jobs:
     needs: setup
     strategy:
       fail-fast: false
-      matrix:
-        browser: [chromium, webkit]
 
     steps:
       - name: Checkout repository
@@ -208,13 +206,11 @@ jobs:
         run: pnpm build --turbo
         # uses Turbopack, much faster and safe enough for a test pipeline
 
-      - name: Install Browser '${{ matrix.browser }}'
-        run: pnpm playwright install --with-deps ${{ matrix.browser }}
+      - name: Install Browser 'chromium'
+        run: pnpm playwright install --with-deps chromium
 
       - name: Run Playwright tests
-        run: pnpm test:no-build --project=${{ matrix.browser }}
-        env:
-          BROWSER_TYPE: ${{ matrix.browser }}
+        run: pnpm test:no-build
 
       - name: Print Final Docker Compose logs
         if: always()