feat(ui): symmetric Connect button + clear error feedback for both sides

[VirusAlex]

Summary

Two complaints from the v0.4.0 live test:

1. Local-token field had no Connect button — user had to press Enter and hope. Peer side has a button. Asymmetric and confusing.
2. Both sides silently swallowed connect failures. Wrong token? Wrong URL? Down peer? Mistyped port? UI just said "disconnected" / "no peer" forever, no hint at what went wrong.

Fix

Symmetric Connect button

Local token field gets the same Connect button the peer side has. Enter still works for power users; click works for everyone else. Both forward to the same connectLocal() entry point. The legacy applyLocalToken() / checkLocal() names are kept as aliases so any external code that called them still works.

4-state status + inline error

Both sides now track a idle / connecting / ok / error status and a localError / peerError string. The status pill cycles through these; the error string sits inline next to it (full text in the title attribute, ellipsis in the chip).

Status	Pill	Pill class
idle	"disconnected" / "no peer"	(neutral)
connecting	"connecting…"	(neutral)
ok	"connected" / "peer ok"	.ok
error	"error" + inline error text	.bad

Classified error messages

HTTP	Message
401	401 — invalid token
403	403 — forbidden
404	404 — endpoint not found (check URL points at NetCopy)
5xx	5xx — peer server error
fetch reject	unreachable (host down, wrong port, or http/https mismatch)

The fetch-reject hint covers the common causes browsers refuse to disclose concretely (CORS / scheme mismatch / unreachable / refused all surface as the same opaque "TypeError: Failed to fetch").

Misc

• Connect button is disabled while a request is in flight; its label becomes … so the user knows something's happening on slow networks.
• Enter in any input field on a side triggers Connect for that side.

Test plan

• Local mvn compile clean; ArchUnit 8/8.
• CI green.
• Manual: paste correct local token + click Connect → "connected".
• Manual: paste wrong local token → "error" + 401 — invalid token.
• Manual: peer URL http://nonexistent:9999 → "error" + unreachable (host down, …).
• Manual: peer URL http://google.com → "error" + 404 — endpoint not found ….

🤖 Generated with Claude Code