fix(client): fail fast on boot/frontend fallbacks (P4)

- MapService load failure is no longer swallowed.
- Character creation fails on unknown selected class instead of ?? 1.
- No prefilled 'Wanderer'/'password' credentials; empty account name is an
  error; unknown MCP class id throws instead of ?? 'so'.
- CommandMapper no longer sends BeltUse with fabricated itemId slot+1.

src/client/frontend/flow-state.ts

@@ -253,8 +253,9 @@ export function createInitialFlowState(initialRoster: RosterCharacter[] = []): B
     stageTicks: 0,
     globalTick: 0,
 
-    accountName: 'Wanderer',
+    // No prefilled credentials: the user (or a configured bot) must type them.
-    accountPassword: 'password',
+    accountName: '',
+    accountPassword: '',
     loginError: null,
 
     campfireClasses,
@@ -338,7 +339,8 @@ export function cancelConnecting(state: BnetFlowState): void {
 export function submitBattleNetLogin(state: BnetFlowState): boolean {
   if (state.stage !== 'bnet_login') return false
   if (!state.accountName.trim()) {
-    state.accountName = 'Wanderer'
+    state.loginError = 'Account name required'
+    return false
   }
   state.loginError = null
   state.stageTicks = 0
@@ -589,7 +591,8 @@ export function charSummaryToRosterCharacter(summary: {
   readonly hardcore: boolean
   readonly ladder: boolean
 }): RosterCharacter {
-  const classId = CHAR_CLASS_ID_TO_CAMPFIRE[summary.charClass] ?? 'so'
+  const classId = CHAR_CLASS_ID_TO_CAMPFIRE[summary.charClass]
+  if (!classId) throw new Error(`MCP character "${summary.name}" has unknown class id ${summary.charClass}`)
   const spec = CAMPFIRE_CLASS_SPECS[classId]
   return {
     id: `${spec.heroToken}-${summary.name}`,

src/client/input/command-mapper.ts

@@ -634,18 +634,9 @@ export class CommandMapper {
     if (event.key === '1' || event.key === '2' || event.key === '3' || event.key === '4') {
       const slotIndex = Number(event.key) - 1
       event.preventDefault?.()
-      if (this.hudModel) {
+      // BeltUse needs the real server item id of the potion, which only HudModel (ClientWorld) knows.
+      if (!this.hudModel) return false
       this.hudModel.useBeltSlot(slotIndex, Boolean(event.shiftKey))
-      } else {
-        this.dispatchRateLimited(
-          {
-            type: 'BeltUse',
-            itemId: slotIndex + 1,
-            shift: Boolean(event.shiftKey),
-          },
-          nowMs,
-        )
-      }
       return true
     }
 

src/client/main.ts

@@ -193,11 +193,12 @@ async function bootPlayApp(): Promise<PlayAppController> {
   })
   await hudManager.load('/ui')
 
-  let mapService: MapService | null = null
+  // Fail fast: without the packed act tiles there is no 1.13c map (automap, collision, picking).
+  let mapService: MapService
   try {
     mapService = await MapService.fromPackedUrl('/act-packs/tiles')
-  } catch {
+  } catch (err) {
-    mapService = null
+    throw new Error(`MapService: failed to load /act-packs/tiles: ${err instanceof Error ? err.message : String(err)}`)
   }
 
   // Kick off asynchronous GPU atlas preloading for WebGL2 world rendering
@@ -468,9 +469,11 @@ async function bootPlayApp(): Promise<PlayAppController> {
           dz: 5,
           as: 6,
         }
+        const classId = st.selectedClass === null ? undefined : classMap[st.selectedClass]
+        if (classId === undefined) throw new Error(`Unknown selected class ${String(st.selectedClass)}`)
         const chars = await session.createCharacter({
           name,
-          classId: classMap[st.selectedClass] ?? 1,
+          classId,
           hardcore: st.isHardcore,
           expansion: st.isExpansion,
           ladder: st.isLadder,