fix(mobile): fix agent avatars on mobile

[brow]

What changed?

• Add a shared mobile avatar renderer for percent-encoded and base64 SVG data URIs, raster data URIs, and network URLs.
• Migrate mobile avatar surfaces to the shared renderer with centered fallbacks.
• Retain parsed raster bytes across rebuilds to avoid decode flicker and image-cache churn.

Why?

Fixes BOT-1306. Agent emoji avatars use percent-encoded SVG data URIs, which Flutter's NetworkImage cannot render, leaving blank avatar circles on mobile even though browser-based desktop clients display them.

Before / After

Before	After

Real Buzz mobile DM on the same simulator and account, before and after the fix, using raccoon’s live 🦝 profile avatar. Emoji avatars are rendered as centered native text on mobile, matching desktop alignment.

How is it tested?

• flutter analyze
• Full mobile test suite: 451 passed, 1 existing skip
• Added focused regression coverage for the raccoon SVG URI, base64 SVG, malformed data, fallback alignment, and raster rebuild stability
• Deep Claude review loop completed with no remaining findings

[brow]

@codex review

[brow]

@builderbot review

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[wesbillman]

Reviewed the full diff. Solid fix: single shared AvatarImage/AvatarImageContent replaces ~10 hand-rolled avatar variants, parses data URIs once (cached across rebuilds), and every failure path (malformed base64, invalid UTF-8, network error) lands on the centered fallback instead of a blank circle. Migration coverage is complete — the only remaining Image.network uses on the branch are non-avatar surfaces (attachments, media viewer, message images, custom emoji). flutter_svg promoted transitive→direct at the already-locked version. CI Mobile (analyze + full suite) green; the regression test pins the exact BOT-1306 percent-encoded SVG URI.