fix: support ipv6 proxy management in admin ui

[gebdalaoli-arch]

This fixes IPv6 proxy management in the Sub2API admin IP/proxy workflow.

Problem:
The admin proxy UI accepted proxy hosts as plain strings, while the backend and helper utilities treated host:port formatting as IPv4-shaped. SOCKS5 IPv6 proxies therefore could be stored or displayed inconsistently, and bracketed IPv6 URLs from imports/checks could fail to match existing proxy entries.

Root cause:
Proxy host normalization and URL formatting did not consistently distinguish the stored host from the URL authority form. IPv6 needs brackets only when combined with a port in URLs, but storage and comparisons should use the canonical bare host form.

Fix:

• Added backend proxy host normalization and URL formatting helpers.
• Normalized IPv6 hosts across create, update, check-exists, import/export, selector, and usage queue paths.
• Added an admin UI IP version selector for IPv4/IPv6 while keeping the existing UI style.
• Added frontend proxy-address helpers for bracketed IPv6 parsing, display, and copy formats.
• Added regression tests for backend normalization/import reuse and frontend IPv6 parsing/formatting.

Verification:

• go test ./internal/service -run "TestNormalizeProxyHost|TestProxyURL|TestProxyURLNormalizesBracketedIPv6Host"
• go test ./internal/handler/admin -run "TestProxyHandlerCreateNormalizesIPv6Host|TestProxyImportDataReusesIPv6ProxyAcrossBracketFormats|TestProxyImportDataReusesIPv6ProxyWhenKeyOnlyUsesBracketedHost|TestImportDataReusesIPv6ProxyKeyAcrossBracketFormats"
• pnpm install --frozen-lockfile
• pnpm typecheck
• pnpm build
• Built asset spot-check for socks5://user:p%40ss%3Aword@[2001:db8::1]:1080 parsing/normalization/URL formatting

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[gebdalaoli-arch]

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[gebdalaoli-arch]

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