Show the nostr pubkey in both encodings, not just npub

[nothing-stops-this-train]

The report

The npub shown in the swapserver gui doesn't match the address shown when a
user clicks "swap" and sees swap providers.

Nothing was actually wrong — they are the same key in two encodings, which
look nothing alike:

Where	Encoding	Looks like
Swap Server tab (before this PR)	bech32 NIP-19	npub1hdd8atxxf9tz284ecgqjpe3eht… (63 chars)
Choose Swap Provider → Pubkey column	raw hex	bb5a7eacc64956251eb9c20120e639ba… (64 chars)

swap_dialog.py sets that column to SwapOffer.server_pubkey — the raw hex
event.pubkey off the nostr event — and keeps the npub only as hidden item
data (ROLE_NPUB), where it is used as the return value and written to
SWAPSERVER_NPUB. It is never rendered. Verified rather than assumed: the npub
decodes back to exactly the hex the provider list displays.

The change

Showing one encoding alone reasonably reads as "the GUI is displaying the wrong
key", so the identity box now shows both, each independently copyable, with a
dimmed one-liner explaining the relationship:

┌─ Nostr identity ────────────────────────────────────┐
│ ■  npub:  npub1hdd8atxxf9tz284ecgqjpe3ehtuc7…  [Copy] │
│    hex:   bb5a7eacc64956251eb9c20120e639baf9…  [Copy] │
│           Same key, two encodings. Takers list hex.   │
└──────────────────────────────────────────────────────┘

The hex was previously only in a tooltip, which is easy to miss.

The identicon is unchanged and remains the fastest visual check that a provider
entry is your server: both sides derive it from the hex via
pubkey_to_q_icon, so the coloured square matches.

Note found while testing

Feeding an npub to pubkey_to_q_icon is not merely a different colour — it is a
hard error, because pubkey_to_rgb_color asserts len == 64. A refactor that
passed the wrong form would crash rather than silently mismatch. There is now a
test pinning that behaviour.

Tests

163 total, green with and without PyQt6 (19 Qt tests, which skip cleanly when it
is absent). New coverage asserts the two rows cannot drift apart:

• the displayed npub must decode back to the displayed hex;
• that hex must equal keypair.pubkey.hex()[2:], the exact value
  SwapOffer.server_pubkey carries;
• the hex Copy button copies the hex, not the npub;
• the identicon is derived from the hex;
• a wallet with no nostr key blanks both rows and makes copying a no-op rather
  than pasting an empty string.

🤖 Generated with Claude Code

https://claude.ai/code/session_013zETFC6MhnsCuxvQDGxqV5