fix(proxy): persist proxy config so it survives saves and reconnects

[sunxiaobin89]

Summary

Proxy settings (type, host, port, username, password) configured on a connection were never persisted. Any save — creating a new connection, editing an existing one, or a failed connect that still saved the connection — silently dropped the proxy config. Reopening the Edit dialog showed empty proxy fields.

Root cause

• The storage model (ConnectionData) had no proxy fields, so no save operation could store them.
• Every save/update path in ConnectionDialog (new connect, edit save) and the connection duplication flow omitted proxy fields.
• All ConnectionData → ConnectionConfig conversions in App.tsx (9 sites) dropped proxy fields, so the edit dialog never received them.

Changes

• src/lib/connection-storage.ts — add proxyType, proxyHost, proxyPort, proxyUsername, proxyPassword to ConnectionData.
• src/lib/connection-config.ts (new) — toConnectionConfig() maps stored connections to the dialog config, carrying proxy settings; replaces 9 duplicated constructions in App.tsx.
• src/components/connection-dialog.tsx — include proxy fields in all 5 save/update calls (SSH, SFTP/FTP/Desktop, edit save); bind value on the proxy username input so saved data displays; normalize proxyType to 'none' when loading a connection without proxy settings.
• src/components/connection-manager.tsx — duplication preserves proxy settings.

Testing

• 12 new tests: toConnectionConfig proxy mapping, storage round-trip, and dialog persistence (edit save, failed connect keeps proxy, edit dialog shows saved values).
• Full suite: 522 tests pass, tsc --noEmit clean, pnpm i18n:check clean, no new lint issues.
• Manually verified: proxy config survives a failed connect and is shown again when editing.

Note: proxy settings are now persisted and round-trip through the edit dialog, but are not yet forwarded to the backend during connection — that is a separate feature, not part of this fix.