Rework e2e tests (#36634)

- Replace the e2e tests initialization with a simple bash script,
removing the previous Go harness.
- `make test-e2e` is the single entry point. It always starts a fully
isolated ephemeral Gitea instance with its own temp directory, SQLite
database, and config — no interference with the developer's running
instance.
- A separate `gitea-e2e` binary is built via `EXECUTABLE_E2E` using
`TEST_TAGS` (auto-includes sqlite with `CGO_ENABLED=1`), keeping the
developer's regular `gitea` binary untouched.
- No more split into database-specific e2e tests. Test timeouts are
strict, can be relaxed later if needed.
- Simplified and streamlined the playwright config and test files.
- Remove all output generation of playwright and all references to
visual testing.
- Tests run on Chrome locally, Chrome + Firefox on CI.
- Simplified CI workflow — visible separate steps for frontend, backend,
and test execution.
- All exported env vars use `GITEA_TEST_E2E_*` prefix.
- Use `GITEA_TEST_E2E_FLAGS` to pass flags to playwright, e.g.
`GITEA_TEST_E2E_FLAGS="--ui" make test-e2e` for UI mode or
`GITEA_TEST_E2E_FLAGS="--headed" make test-e2e` for headed mode.
- Use `GITEA_TEST_E2E_DEBUG=1 make test-e2e` to show Gitea server
output.

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -178,7 +178,14 @@ Here's how to run the test suite:
 | :------------------------------------------ | :------------------------------------------------------- | ------------------------------------------- |
 |``make test[\#SpecificTestName]``            |  run unit test(s)                                        |                                             |
 |``make test-sqlite[\#SpecificTestName]``     |  run [integration](tests/integration) test(s) for SQLite | [More details](tests/integration/README.md) |
-|``make test-e2e-sqlite[\#SpecificTestName]`` |  run [end-to-end](tests/e2e) test(s) for SQLite          | [More details](tests/e2e/README.md)         |
+|``make test-e2e``                            |  run [end-to-end](tests/e2e) test(s) using Playwright    |                                             |
+
+- E2E test environment variables
+
+| Variable                  | Description                                                       |
+| :------------------------ | :---------------------------------------------------------------- |
+| ``GITEA_TEST_E2E_DEBUG``  | When set, show Gitea server output                                |
+| ``GITEA_TEST_E2E_FLAGS``  | Additional flags passed to Playwright, for example ``--ui``       |
 
 ## Translation