Add local file access diagnostics and direct-file E2E coverage

[YUMA-NAGAO]

Summary

• show a popup warning when Chrome local-file access is unavailable, with an extension-settings CTA and manual fallback
• document the required permission toggle and file-tab reload step
• replace the unrelated file-origin switch with Chromium's extension file-access test switch
• add a synthetic MP4 fixture plus direct-file and relative-local-HTML E2E coverage
• update structural validation for the current build output

Testing

• npm ci
• npm run lint
• npm run build:release
• npm test (32 files, 384 tests)
• npm run test:e2e:local (34 structural checks and 6/6 browser checks)
• Chrome for Testing: CTA opened chrome://extensions/?id=<runtime-id> without a tabs permission
• independent diff review and Codex second-opinion review

Manual validation remaining

• Stable Google Chrome: permission-off warning is visible
• Stable Google Chrome: settings CTA is accepted
• Stable Google Chrome: after enabling access and reloading, a direct local MP4 is controllable

Notes

• No manifest permissions changed; in particular, this does not add tabs, activeTab, host_permissions, scripting, or <all_urls>.
• The local-file browser E2E intentionally runs in allowed-access mode via Chromium's extension test switch. Permission-denied and detection-failure UI states are covered by unit tests; the branded-Chrome permission toggle remains a manual check.
• The committed MP4 is a generated, silent black test fixture; no user media or third-party content is included.
• npm audit --audit-level=high reports the same existing 9 advisories (4 high) on upstream/master and this branch; dependency updates are intentionally out of scope.