feat(web): define ChatEvent discriminated union, eliminate all any sites (R9)

Created web/src/lib/types.ts with discriminated unions for SSE event
payloads: ChatEvent (7 variants: session, tool_use, tool_result,
text_delta, text, done, error), ToolCallResult, MessageContent, and
typed data shapes for live events (PlanProposedData, PlanStepEventData,
QuestionRaisedData, QuestionAnsweredData, EntityTouchedData,
HealthChangedData) plus WailsGlobal for the desktop bridge.

Replaced all ~15 `any` sites across 7 files:
- api.ts: Message.content any -> MessageContent | string; removed local
  ChatEvent interface (now imported from types.ts as a discriminated
  union); JSON.parse cast to ChatEvent.
- stores/chat.ts: removed local ToolCallResult interface (imported from
  types.ts, re-exported for backward compat); extractApprovals accesses
  args with typeof guards instead of implicit any access; toChatMessages
  handles string|object Message.content cleanly.
- stores/activity.ts: update_plan_step seq/status extracted via typeof
  guards instead of `as any` casts; toolActivityLabel uses a str() helper
  for safe string extraction from unknown args.
- stores/workspace.ts: applyPlanStepEvent takes PlanStepEventData;
  applyEvent casts data to Record<string, unknown>; switch cases cast to
  typed interfaces (PlanProposedData, QuestionRaisedData, etc.) instead
  of `as any`; applyHealthChanged uses HealthChangedData.
- Config.svelte: (window as any).wails -> typed WailsGlobal cast;
  catch (e: any) -> catch (e: unknown) with instanceof Error check.
- utils.ts: WithoutChild/WithoutChildren `any` -> `unknown`.
- vite.config.ts: authProxy proxy/proxyReq `any` -> ProxyOptions type.

Result: eslint no-explicit-any warnings dropped 12 -> 0. Tests (6/6) and
build pass. VERSION 0.7.10 -> 0.7.11. Plan R9 marked done.

web/src/lib/api.ts

@@ -29,7 +29,7 @@ export interface Message {
   id: string
   session_id: string
   role: string
-  content: any
+  content: MessageContent | string
   created_at: string
 }
 
@@ -102,13 +102,6 @@ export async function answerQuestion(sessionId: string, questionId: string, answ
   return res.ok
 }
 
-export interface ChatEvent {
-  type: string
-  data: any
-  session_id?: string
-  iteration?: number
-}
-
 export function streamChat(
   message: string,
   sessionId: string | null,