feat(proxy): add subscription link support

[chillykidd]

Summary

• Add a subscription URL proxy configuration mode that downloads supported proxy links and uses the selected server as the section outbound.
• Add a LuCI subscription server picker with on-demand loading, persisted selection, and dashboard display support.
• Add runtime subscription parsing for plain/base64 subscriptions while masking subscription fields in diagnostics output.

Test plan

• corepack yarn build
• corepack yarn lint --max-warnings=0
• corepack yarn test --run
• bash -n podkop/files/usr/bin/podkop
• Manual OpenWrt smoke test: load a subscription URL, select a server, save, and verify podkop.main.subscription_proxy_link is persisted

[chillykidd]

[wampir4ik]

Very nice touch