fix(dashboard): unify custom provider icon fallbacks

[ricatix]

Summary

• Add a shared resolveProviderIconInfo helper for provider fallback text/color.
• Use display-name initials for compatible provider fallbacks across Providers, Usage topology, Combo edit, and Add Model modal.
• Cover built-in, OpenAI-compatible, Anthropic-compatible, empty-name, null-name, and unknown-provider cases with unit tests.

Linked issue

• Closes Unify custom provider fallback icons across dashboard #1831

Follow-up

• Optional favicon discovery/cache is tracked separately in Explore favicon discovery for custom provider icons #1830

Validation

• cd tests && NODE_PATH=/tmp/node_modules /tmp/node_modules/.bin/vitest run --reporter=verbose unit/provider-icon-info.test.js
• bun run build
• Full suite checked during implementation; existing unrelated failures remained pre-existing/no regression.