feat: identicons, tap-to-copy fingerprint, recent rooms (Phase 1 backport)

Backport from desktop client to Android:

Identicons:
- New Identicon.kt composable: deterministic 5x5 symmetric Canvas pattern
  from fingerprint hash (same algorithm as desktop identicon.ts)
- Participant list shows identicon + name + tappable fingerprint
- Settings page shows identicon next to fingerprint

CopyableFingerprint:
- Tap any fingerprint text to copy to clipboard with Toast feedback
- Used in participant list and settings page

Recent rooms:
- SettingsRepository: persists last 5 (relay, room) pairs
- CallViewModel: saves on startCall, exposes as StateFlow
- InCallScreen: clickable chips that fill room + select matching server

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/wzp/data/SettingsRepository.kt

@@ -28,6 +28,7 @@ class SettingsRepository(context: Context) {
         private const val KEY_PREFER_IPV6 = "prefer_ipv6"
         private const val KEY_IDENTITY_SEED = "identity_seed_hex"
         private const val KEY_AEC_ENABLED = "aec_enabled"
+        private const val KEY_RECENT_ROOMS = "recent_rooms"
     }
 
     // --- Servers ---
@@ -138,4 +139,33 @@ class SettingsRepository(context: Context) {
     fun saveSeedHex(hex: String) {
         prefs.edit().putString(KEY_IDENTITY_SEED, hex).apply()
     }
+
+    // --- Recent rooms ---
+
+    data class RecentRoom(val relay: String, val room: String)
+
+    fun addRecentRoom(relay: String, room: String) {
+        val rooms = loadRecentRooms().toMutableList()
+        rooms.removeAll { it.relay == relay && it.room == room }
+        rooms.add(0, RecentRoom(relay, room))
+        if (rooms.size > 5) rooms.subList(5, rooms.size).clear()
+        val arr = JSONArray()
+        rooms.forEach { arr.put(JSONObject().apply { put("relay", it.relay); put("room", it.room) }) }
+        prefs.edit().putString(KEY_RECENT_ROOMS, arr.toString()).apply()
+    }
+
+    fun loadRecentRooms(): List<RecentRoom> {
+        val json = prefs.getString(KEY_RECENT_ROOMS, null) ?: return emptyList()
+        return try {
+            val arr = JSONArray(json)
+            (0 until arr.length()).map { i ->
+                val o = arr.getJSONObject(i)
+                RecentRoom(o.getString("relay"), o.getString("room"))
+            }
+        } catch (_: Exception) { emptyList() }
+    }
+
+    fun clearRecentRooms() {
+        prefs.edit().remove(KEY_RECENT_ROOMS).apply()
+    }
 }