fix(web): match Sentinel application icon fallbacks

[BK1031]

• Replace the generic application glyph with Sentinel-style initial tiles
• Match Sentinel’s pink-to-purple fallback gradient and typography
• Fall back to the initial when a configured application icon fails to load

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-31T23:05:23.934895Z	1fad587	PR opened

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 1fad5874c4

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[chatgpt-codex-connector]

Keep the application icon hidden from assistive technology

When a screen-reader user encounters any current caller, the icon is immediately followed by the same application name, so alt={name} causes configured icons to announce the name twice; the textual initial fallback is likewise exposed. Keep this visual decorative (for example, hide the wrapper from assistive technology and restore an empty image alt) so buttons and identity rows retain a single accessible label.

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