fix: recognize flow.google.com and surface upload bridge errors

[ballz921]

Summary

Related to #10 — partial remediation, not a claim that the new Flow generation API works end to end.

• Add https://flow.google.com/* to the extension's host permissions, content script matches, and web-accessible resource matches, while retaining legacy Flow support.
• Reuse new/legacy Flow tab patterns and check exact HTTPS hostnames / legacy Flow paths before content-script injection.
• Return createdTab from the fresh-tab path instead of the undefined retryTabs[0].
• Preserve upload response status and actionable scalar error messages instead of collapsing top-level bridge errors to Unknown. Do not serialize response objects into logs.
• Add regression tests and document remaining compatibility limitations.

Observed failure

An installation at 206285a recorded these extension errors:

CAPTCHA_FAILED: Cannot access contents of url "https://flow.google.com/". Extension manifest must request permission to access this host.
retryTabs is not defined

The Python upload path reduced these errors to Image upload failed: Unknown, then the local API returned HTTP 500. That local 500 is not proof that Google's upload endpoint returned 500.

Validation

From flow-agent/:

python3 -m pytest -q tests/test_extension_flow_urls.py tests/test_media_storage.py
# 20 passed

WS_PORT=0 HTTP_PORT=0 python3 -m pytest -q
# 84 passed

node --check ../flow-extension/background.js
git diff --check

• Tests were introduced before the implementation; initial focused run reproduced missing host/tab support and lost upload error details.
• A separate local Node VM probe executed the actual URL predicate against 10 accepted/rejected URLs and exercised fresh-tab return plus post-redirect injection with mocked Chrome APIs; all passed. That probe is supplementary, not a live Chrome test.
• The first full run inside a restricted sandbox could not bind lifecycle-test sockets. The complete suite was rerun successfully outside that sandbox with ephemeral test ports; no production service was restarted.
• Manifest/background regression tests in this PR are source contracts, not browser end-to-end tests.

Deliberately out of scope

No cookie/batchexecute migration, authentication changes, CAPTCHA disabling/bypass, Origin/Referer changes, REST endpoint changes, or callback-secret handling changes. New-site token capture, CAPTCHA execution and live upload/generation remain unverified and may require follow-up work described in #10. This PR does not close that broader issue.

[santoshray02]

Reproduced both errors on a live deployment (backend 2.0.5 on a headless VPS, extension in a remote Chrome) on 2026-09-08:

• CAPTCHA_FAILED: Cannot access contents of url "https://flow.google.com/" with a flow.google.com tab open
• retryTabs is not defined on the fresh-tab path with no Flow tab open

This PR's manifest and background.js changes are exactly what fixes those two failures; I had hand-patched the same lines locally before finding this and reverted in favour of it. The isFlowUrl() predicate is cleaner than the old regex.

Two notes for whoever merges:

1. As the description says, this does not make generation work on flow.google.com. #10 documents that the new site uses cookie auth, no grecaptcha, and a batchexecute RPC instead of the aisandbox-pa REST call. Worth keeping #10 open or re-linking it, since it is closed.
2. https://labs.google/fx/tools/flow still serves the full app with no server-side redirect as of today, so with this PR the legacy path remains the working one when a labs.google tab is in the foreground.

Approving on the strength of the reproduced fix; the #10 port is separate work.