feat: pin TCP/QUIC listen ports + visible error banner

Two related improvements requested while testing the private fork:

1. Custom TCP/QUIC listen ports (SidecarConfig.tcpListenPort,
   .quicListenPort). With ephemeral ports, the daemon's peer-listen
   port changes at every restart, which makes port-forwarding on a
   home router useless after the first daemon stop. Pinning these
   keeps the same port across restarts so other private-network
   nodes can dial in reliably.

   Backend uses the configured port if Some, falls back to
   pick_port_skip otherwise. Frontend exposes two inputs in
   Settings → Daemon configuration with a help line explaining
   when to fill them.

2. Daemon-failure banner in App.vue. The previous behaviour was
   silent: a click on \"Start daemon\" that hit a backend error
   only flipped the sidebar dot to red, with no message visible.
   Now an inline banner at the top of the main content area shows
   the error, plus a \"Go to Settings\" shortcut when the message
   mentions a network config issue.

CHANGELOG.md

@@ -6,6 +6,11 @@ This fork tracks [`syoul/Mycell-UI`](https://git.open.us.org/syoul/Mycell-UI) (t
 
 ## [Unreleased]
 
+### Added
+- `SidecarConfig.tcpListenPort` and `SidecarConfig.quicListenPort` (Option<u16>): pin the daemon's peer-listen ports across restarts so they can be reliably port-forwarded on a home router. `None` keeps the previous ephemeral behaviour.
+- Two input fields in **Settings → Daemon configuration** to expose them, with an explanation of when to set them.
+- App.vue surfaces a top banner when `phase === 'error'` with the daemon error message and a shortcut "Go to Settings" button.
+
 ## [0.1.0] — 2026-04-27
 
 Forked from `Mycell-UI@5229e2c` ("feat(packaging): pre-spawn cleanup wrapper for clean restarts").