ux(quick-connect): hint custom server username path (#23)

[dzianisv]

Summary

• Adds a one-line hint under the Quick Connect password field: states username defaults to `opencode` and links to Advanced options for servers with a custom `OPENCODE_SERVER_USERNAME`.
• Tap on "Advanced options" inside the hint switches the screen to advanced mode (same handler as the existing link).

Why

Per #23 — the Quick Connect auth fix (defaulting username to `opencode`) covers most users, but custom-username servers were silently unreachable from the simple flow. A small hint surfaces the escape hatch without bloating Quick Connect.

Changes

• `app/connection/add.tsx`: hint under the password input + new `usernameHint`/`usernameHintLink` styles.

Verification

• `npx tsc --noEmit` — clean.
• Manual UI check pending on next CUA / device pass.

Closes #23