feat(desktop): make the PAC port a local-ports setting

[loss-and-quick]

Summary

Make the desktop PAC-server port a user setting (localPacPort, default 10811), symmetric with localSocksPort/localHttpPort — closing the "fixed PAC port" note from #218/#65. The default constant is single-sourced in kasumi-core and exported to the frontend through the generated defaults; pac::start now binds the port it is given (resolved in DesktopPlatform beside the http port — the Platform trait is untouched). The settings UI shows the field in Local ports, desktop-only (the PAC mode doesn't exist on Android).

Affected layer

• frontend/ — React Web UI
• crates/ · src-tauri/ — Rust core / backend / Tauri desktop
• module/ — Android installable zip (thin launcher over the Rust daemon)
• scripts/ — build / release helpers
• CI / .github/
• Docs only

Verification

Web UI (frontend/):

• bun run check — Biome lint + format clean
• bun run test — vitest green (69 passed)
• bun run build — tsc -b + vite build succeed
• bun run check:i18n — locale dictionaries in sync (652 keys × 8 locales)

Rust (crates/ · src-tauri/):

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace — one failure: net::tests::tcp_ping_fails_on_unreachable_host, the known environment flake (an active VPN tun on the dev machine captures the TEST-NET-1 connect); fails identically on main
• Codegen drift: cargo run -p kasumi-desktop --bin codegen leaves git clean

Checklist

• Title is a scoped Conventional Commit; commits are logically split
• No build artifacts committed (module/bin/<abi>/, geoip/geosite, built module/webroot/, src-tauri/gen/ — all gitignored on purpose)
• Generated frontend/src/generated/ was regenerated from Rust, not hand-edited
• If user-visible strings changed: i18n/en.ts and every locale file updated (one label key, settings.pac, ×8)
• Renames touching the project id were grepped in all case forms (kasumi-proxy, Kasumi Proxy, camelCase)

Notes for reviewers

• The label key is settings.pac ("PAC" in every locale), mirroring the sibling acronym labels settings.socks/settings.http rather than a longer sentence-style key.
• The Local-ports row now holds three number fields on desktop (two elsewhere); not visually eyeballed in a running app — flag if the row wraps badly on narrow widths.
• No port-collision validation (socks/http/pac/force): the pre-existing fields have none either; the PAC bind failure path (port taken → OS left un-proxied + logged error) already covers the misconfiguration at runtime.