feat(direct-call): call history, recent contacts, deregister button

Persistent JSON-backed call history for the direct-call screen so users
can see what they've placed / received / missed and dial back with one
click. Also fixes two small latent UX issues reported alongside.

Backend (Rust)
- new crate/module desktop/src-tauri/src/history.rs: thread-safe in-
  process store (OnceLock<RwLock<Vec<CallHistoryEntry>>>) backed by
  <APP_DATA_DIR>/call_history.json. Atomic writes via temp+rename. Max
  200 entries, FIFO pruning. CallDirection { Placed, Received, Missed }.
- Log hooks in the signal loop + commands:
    * place_call     → Placed entry (with target fingerprint)
    * DirectCallOffer → Missed entry up front; upgraded to Received
                        inside answer_call when accept_mode != Reject
                        via history::mark_received_if_pending(call_id).
                        If user rejects or never answers, it stays Missed.
- New Tauri commands:
    * get_call_history()     → all entries, newest first
    * get_recent_contacts()  → unique peers by fp, newest interaction first
    * clear_call_history()   → wipes JSON + in-memory
    * deregister()           → tears down signal transport + endpoint
  Backend emits `history-changed` events so the UI can live-refresh
  without polling.

Frontend (main.ts + index.html + style.css)
- Direct-call panel now has:
    * Recent contacts chip row (top 6 unique peers). Click a chip → dial.
    * Call history list (up to 50 rows). Direction icon (↗ placed, ↙
      received, ✗ missed), peer alias/fp, relative timestamp, callback
      button. Both click handlers populate target-fp and fire place_call.
    * Deregister button in the "registered" header — calls the new
      deregister command, tears down the signal transport, returns the
      UI to the pre-register state.
    * Clear-history link in the history header.
- Subscribes to `history-changed` events so the list updates the moment
  the backend logs a new entry. Also refreshed on register + after a
  clear.
- Nothing is rendered until there is data — empty sections stay hidden.

Tasks #20 + #21 (small UX items bundled in)
- Default room "general" for new installations: the html input value
  attribute is now "general" and loadSettings() defaults match. Existing
  users' localStorage still wins.
- Random alias on desktop: already latent but confirmed working — the
  startup IIFE at main.ts:374 calls get_app_info() and prefills the
  alias input from derive_alias(seed) when the input is empty. No code
  change needed, just verified it flows through the same path as the
  Android client.

Known follow-ups (deferred to step 6 polish)
- Call duration tracking (currently all entries have no duration field)
- Hangup signal from an unanswered incoming should emit history-changed
  so the missed state is visible even when the user never tapped accept
- Android UI layout fit-check on the smaller Nothing screen

desktop/src/style.css

@@ -890,3 +890,142 @@ button.primary:disabled { opacity: 0.5; cursor: not-allowed; }
 .mode-btn:hover:not(.active) {
   background: var(--surface2);
 }
+
+/* ── Direct call history + contacts ── */
+
+.direct-registered-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  margin-bottom: 4px;
+}
+
+.secondary-btn.small {
+  padding: 4px 10px;
+  font-size: 11px;
+}
+
+.history-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin: 12px 0 4px 0;
+  font-size: 11px;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  color: var(--dim);
+}
+
+.link-btn {
+  background: none;
+  border: none;
+  color: var(--dim);
+  font-size: 11px;
+  cursor: pointer;
+  padding: 2px 6px;
+  text-decoration: underline;
+}
+.link-btn:hover { color: var(--text); }
+
+.history-list {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  max-height: 180px;
+  overflow-y: auto;
+}
+
+/* Recent contacts — horizontally wrapping chips */
+#recent-contacts-list {
+  flex-direction: row;
+  flex-wrap: wrap;
+  max-height: none;
+}
+
+.contact-chip {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  background: var(--surface2);
+  border: 1px solid var(--surface2);
+  color: var(--text);
+  border-radius: 16px;
+  padding: 4px 10px;
+  font-size: 12px;
+  cursor: pointer;
+  transition: all 0.15s;
+}
+.contact-chip:hover {
+  background: var(--accent);
+  border-color: var(--accent);
+  color: white;
+}
+.contact-dot {
+  width: 6px;
+  height: 6px;
+  border-radius: 50%;
+  background: var(--green);
+}
+.contact-chip .contact-label {
+  max-width: 120px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+/* Full history rows */
+.history-row {
+  display: grid;
+  grid-template-columns: 20px 1fr auto;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 8px;
+  background: var(--surface);
+  border-radius: 6px;
+  font-size: 12px;
+}
+.history-row.dir-placed   .history-dir { color: var(--accent); }
+.history-row.dir-received .history-dir { color: var(--green); }
+.history-row.dir-missed   .history-dir { color: var(--red); }
+.history-dir {
+  text-align: center;
+  font-weight: 700;
+}
+.history-meta {
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  overflow: hidden;
+}
+.history-peer {
+  color: var(--text);
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.history-time {
+  color: var(--dim);
+  font-size: 11px;
+}
+.history-call-btn {
+  background: var(--surface2);
+  border: 1px solid var(--surface2);
+  color: var(--text);
+  padding: 4px 10px;
+  border-radius: 4px;
+  font-size: 11px;
+  cursor: pointer;
+  transition: all 0.15s;
+}
+.history-call-btn:hover {
+  background: var(--accent);
+  border-color: var(--accent);
+  color: white;
+}
+
+/* Speaker routing button (non-muted earpiece state should not look red) */
+#spk-btn.speaker-on .icon {
+  color: var(--accent);
+}