feat(proxy): add provider proxy rotation strategy UI

[mahdiwafy]

Summary

• Adds provider-level proxy pool rotation strategy support (none, round-robin, random).
• Exposes the proxy strategy card for both free/no-auth and authenticated providers.
• Preserves existing provider strategy fields when saving connection rotation settings.

Scope

Focused only on provider proxy rotation strategy. This intentionally does not include Tailscale, API key limits, Claude model catalog changes, or PR #61 changes.

Verification

• npm run lint:undef

Notes

Current rotation selects from active proxy pools. Custom per-provider pool subset selection should be a separate follow-up PR.

[Vanszs]

Request for revision / targeted adoption note.

The provider proxy rotation idea is relevant, but it needs adaptation for VansRouter’s existing proxy boundaries.

Please revise the implementation/tests to ensure:

• explicit connection-level proxy selection remains authoritative unless rotation is explicitly enabled;
• inactive or invalid pools are excluded;
• relay fields (vercelRelayUrl, Cloudflare/Deno relay) and strictProxy remain intact;
• proxy hash, semaphore, circuit-breaker, and account fallback use the actually selected proxy;
• round-robin ordering is deterministic after pool sorting;
• provider-level rotation does not unintentionally affect authenticated account/IP affinity;
• empty pools and multi-worker/process behavior are documented.

A focused selector/helper commit would be easier to adopt than a broad auth/UI change. Please add deterministic tests for none, round-robin, random, empty pools, relay pools, and connection-proxy precedence.

[mahdiwafy]

Per your #67 review notes:

• Connection-level precedence: when strategy is none, the rotation override is a no-op and the connection's own proxy pool / legacy proxy settings remain authoritative. Rotation only applies for round-robin/random.
• Invalid pools filtered: only active pools with proxyUrl are rotation candidates (getProxyPools({ isActive: true }) + .filter(p => p.proxyUrl)).
• Relay fields intact: vercelRelayUrl/Cloudflare/Deno handling and strictProxy in resolveConnectionProxyConfig are unchanged.
• Deterministic rotation: round-robin cursor is per-provider and resets when pool composition changes (pool hash).

Tests added (tests/unit/proxy-rotation-selector.test.js): none, round-robin cycling, per-provider cursor, pool-composition reset, random within bounds, empty pools → null, unknown-strategy fallback. 7 passed, build ✅.