test: activate the full e2e (browser-driving) suite + add fetch --force

The 138 @pytest.mark.e2e tests were doubly inactive: deselected by addopts AND
skipped without a cached binary — and 3 of the 6 per-file firefox_binary
fixtures silently ignored INVPW_BINARY_PATH, so they'd test whatever was cached
even when you pointed the suite elsewhere (a false-confidence trap).

- Centralize firefox_binary into conftest.py (env INVPW_BINARY_PATH → cache →
  skip); delete the 6 duplicates. Unify test_webrtc_realness onto the same env.
- scripts/run_e2e.py: one command that runs ALL e2e against a chosen binary,
  with reruns so an under-load interaction flake (dblclick/hover pass 3/3 in
  isolation) self-heals while a real break fails every attempt. The webrtc e2e
  fake a TCP-only SOCKS locally, so the suite is offline. This is the MANDATORY
  pre-release browser gate (local — hosted runners are too interaction-flaky).
- Running the suite against firefox-9 surfaced a real gap: `invisible_playwright
  fetch --force` was unrecognized (the subparser took no args) though the e2e
  test + docstring expect it. Implement it: drop the cached version dir, refetch.
- Add pytest-rerunfailures + playwright to the dev extras.

Baseline against firefox-9: 136 passed, 1 skipped (linux_only on win host),
1 was the --force gap now fixed.

pyproject.toml

@@ -30,7 +30,7 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
-dev = ["pytest>=7", "pytest-mock>=3", "responses>=0.24", "build>=1"]
+dev = ["pytest>=7", "pytest-mock>=3", "responses>=0.24", "build>=1", "pytest-rerunfailures>=14", "playwright>=1.40"]
 
 [tool.pytest.ini_options]
 markers = [