Fix blank review avatars on app detail page

[mdmohsin7]

Problem

Reviewer avatars on the app detail page (Apps → app → Reviews) render as blank grey circles.

Avatars are not stored anywhere — neither the AppReview model nor the backend includes a photo field. They were generated at render time from a third-party hobby service, avatar.iran.liara.run. That service is currently unreachable: every request hangs (HTTP 000, the TCP handshake never completes). Because a hung connection never raises an error, the widget's initial-letter errorBuilder fallback never fired, leaving the grey placeholder circle on screen indefinitely.

Fix

Drop the external dependency entirely. New ReviewAvatar widget renders a fully local initials avatar — a deterministic background color derived from the reviewer's uid/username plus their first initial. No network call, works offline, can't regress on a third-party outage.

Wired into both call sites:

• Full reviews list (reviews_list_page.dart)
• Detail-page review preview (app_detail.dart)

The now-unused _getAvatarUrl helpers were removed from both files.

Verification

No simulator/device available in the build env, so verified programmatically:

• flutter analyze clean on all touched files.
• New widget test (test/widgets/review_avatar_test.dart), 5/5 pass — asserts the correct initial renders, falls back to A on an empty name, emits zero Image widgets (never touches the network), and produces a stable color per seed.

[cubic-dev-ai]

2 issues found across 4 files

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Fix all with cubic | Re-trigger cubic</sub>