fix: remove empty auth env placeholders from anthropic-thirdparty and openrouter presets

[eriksun421]

Fixes #241

Root Cause

anthropic-thirdparty and openrouter presets were storing empty auth key placeholders ({"ANTHROPIC_API_KEY":""} or {"ANTHROPIC_AUTH_TOKEN":""}) in extra_env. This caused two problems:

1. inferAuthStyleFromLegacy() in provider-catalog.ts detects auth style by checking if ANTHROPIC_AUTH_TOKEN key exists in extra_env (regardless of value), so an empty placeholder could cause the wrong auth style to be inferred.
2. The empty entries were persisted to DB and shown in the UI after save/reopen, confusing users into thinking their credentials were being cleared.

Fix

• Remove the empty auth key placeholders from the preset definitions in ProviderManager.tsx and ProviderForm.tsx
• On save for anthropic-thirdparty, strip any ANTHROPIC_API_KEY / ANTHROPIC_AUTH_TOKEN keys from extra_env instead of injecting empty ones — auth injection is already handled correctly via the api_key field in toClaudeCodeEnv()

Testing

1. Add an Anthropic Third-party API provider with a valid base URL and API key
2. Save and reopen — no empty auth env vars should appear in the UI
3. Send a message — Claude Code subprocess should start successfully

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