Retire DOT-HEZ bridge from the Bridge screen

The DOT<->HEZ bridge backend (the legacy single-key bot) is being
permanently decommissioned - it had zero multisig protection and no
liquidity gate at all on that leg (unconditionally "active" regardless
of pool state, the single least-safe path in the whole bridge). Once
USDT bridging is solid, HEZ is reachable indirectly (bridge USDT, swap
for HEZ within Pezkuwi's own ecosystem) without a dedicated native
bridge that would need its own from-scratch security work (native
currency has no Assets.approve_transfer-style delegation to reuse).

- BridgePair/BridgeDirection collapsed to USDT-only (USDT_TO_WUSDT/
  WUSDT_TO_USDT) - kept the pair/pairOptions/picker structure itself
  since it costs nothing and is the natural seam for a future pair,
  rather than hardcoding a single value inline.
- Removed the now-dead DOT/HEZ exchange-rate fetching (CoinGecko
  polling, fallback rate) and liquidity-gate logic entirely, not just
  the unreachable enum branches.
- Removed the separate "Bridge DOT ↔ HEZ" entry from the asset
  selector bottom sheet (BuySellSelectorMixin) - both it and "Bridge
  USDT" opened the exact same screen with no preset, so keeping a
  DOT-HEZ-labeled entry pointing at a USDT-only screen would have been
  actively misleading, not just redundant.
- bridge_title was the generic toolbar/picker-sheet title but hardcoded
  "DOT ↔ HEZ Bridge" regardless of pair - fixed to "USDT Bridge".
- Removed now-unreferenced strings (wallet_asset_bridge,
  bridge_pair_dot_hez, bridge_rate_format(_reverse),
  bridge_hez_to_dot_note/warning/blocked) from the base strings.xml.

Confirmed via research: feature-bridge-api/feature-bridge-impl modules
exist but are unrelated orphaned scaffolding, not depended on by :app,
zero code overlap with this screen - correctly out of scope here.

feature-assets/src/main/res/layout/fragment_bridge.xml

@@ -113,7 +113,7 @@
                             android:layout_height="wrap_content"
                             android:textColor="@color/text_primary"
                             android:textSize="14sp"
-                            tools:text="1 DOT = 3.00 HEZ" />
+                            tools:text="1:1 (fee 0.1%)" />
 
                     </LinearLayout>
 
@@ -162,7 +162,7 @@
                             android:layout_height="wrap_content"
                             android:textColor="@color/text_primary"
                             android:textSize="14sp"
-                            tools:text="0.1 DOT" />
+                            tools:text="1.0 USDT" />
 
                     </LinearLayout>
 
@@ -170,7 +170,7 @@
 
             </com.google.android.material.card.MaterialCardView>
 
-            <!-- Warning for HEZ to DOT / USDT liquidity -->
+            <!-- Warning for USDT liquidity -->
             <io.novafoundation.nova.common.view.AlertView
                 android:id="@+id/bridgeWarningAlert"
                 android:layout_width="match_parent"