feat(providers): custom icon URL for compatible providers

[whale9820]

What

Lets users set their own icon for OpenAI/Anthropic-compatible (custom) providers via an optional Icon URL field in the Add and Edit modals. The image shows up in the provider card grid and the provider detail header, falling back to the existing text badge (OC/AC) if unset or if it fails to load.

/cc @decolua — quick note on why this is trivial to land: see "Why this is a 1-click add" below.

Why

Custom providers currently can only show a hardcoded OC/AC text badge, or a non-existent bundled asset (/providers/<id>.png → 404 → text fallback). There was no way to brand a self-hosted / custom endpoint. This adds the missing optional iconUrl field end-to-end.

Changes (8 files, +43/-4)

Data layer — iconUrl is stored in the provider node's existing JSON data blob, so no schema migration and it round-trips through backups automatically:

• src/lib/db/repos/nodesRepo.js — createProviderNode now persists iconUrl (was dropped by the hardcoded field list; updateProviderNode already spread extras).
• POST /api/provider-nodes (route.js) — destructure + pass iconUrl through to all three node types (openai-compatible, anthropic-compatible, custom-embedding).
• PUT /api/provider-nodes/[id] — pass iconUrl through in updates.

UI — one optional Input ("Icon URL") in each modal:

• AddCompatibleModal.js — field + send on create.
• EditCompatibleNodeModal.js — field + send on save.

Render — prefer iconUrl as the icon src:

• providers/page.js — both card components + the custom-node mappings now carry iconUrl.
• providers/[id]/page.js — header icon; uses ProviderIcon (plain <img>) for the remote case to avoid next/image remotePatterns config, keeps next/image for bundled assets.
• shared/components/index.js — export the existing ProviderIcon (it already supported src + text fallback).

Why this is a 1-click add

The infrastructure already existed — ProviderIcon has supported a remote src with a text fallback since day one, and provider nodes are stored as a flexible JSON blob that already round-trips arbitrary fields through updateProviderNode. The only reason custom icons didn't work was a field whitelist: the API routes and createProviderNode explicitly enumerated {name, prefix, apiType, baseUrl} and silently dropped everything else, and the render sites hardcoded src={/providers/\${id}.png}. So this PR is almost entirely "stop dropping the field + add one text input + src={iconUrl || existing}" — no new abstractions, no DB migration, no image upload/storage, no config. The hardest decision (Next.js remote image config) is sidestepped by reusing the existing <img>-based ProviderIcon for remote URLs.

Verification

• node --check passes on all 8 edited files.
• No new dependencies.
• Backward compatible: iconUrl is optional; unset nodes behave exactly as before (text badge fallback).

Out of scope (deliberately)

• No image upload/storage (URL only) — keeps the PR dependency-free. Upload can be a follow-up if desired.
• No next/image remotePatterns changes — remote icons go through ProviderIcon's plain <img>.