feat: display inference statistics in chat UI

Add InferenceStats proto message and InferenceStatsPanel component that
displays token counts, throughput, and context window utilization as a
collapsible panel below assistant messages after orchestration completes.

Closes #43

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

implementation-plans/issue-043.md

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+# Issue #43: Display Inference Statistics in Chat UI
+
+**Status:** IMPLEMENTING
+**Issue:** [#43](https://git.shahondin1624.de/llm-multiverse/llm-multiverse-ui/issues/43)
+**Branch:** `feature/issue-43-inference-stats`
+
+## Overview
+
+Add a collapsible UI panel that displays LLM inference statistics (token counts, context window utilization, throughput) below assistant messages after orchestration completes.
+
+## Phases
+
+### Phase 1: Proto Types
+
+**Files:**
+- `proto/upstream/proto/llm_multiverse/v1/common.proto` — Add `InferenceStats` message
+- `proto/upstream/proto/llm_multiverse/v1/orchestrator.proto` — Add optional `inference_stats` field to `ProcessRequestResponse`
+
+**InferenceStats message fields:**
+- `prompt_tokens` (uint32) — tokens in the prompt
+- `completion_tokens` (uint32) — tokens generated
+- `total_tokens` (uint32) — sum of prompt + completion
+- `context_window_size` (uint32) — model's maximum context length
+- `tokens_per_second` (float) — generation throughput
+
+**Then regenerate types:** `npm run generate`
+
+### Phase 2: Orchestration State
+
+**Files:**
+- `src/lib/composables/useOrchestration.svelte.ts` — Extract `inferenceStats` from response, expose via store getter
+
+### Phase 3: InferenceStatsPanel Component
+
+**Files:**
+- `src/lib/components/InferenceStatsPanel.svelte` — New component
+
+**Design:**
+- Follow `<details>` pattern from FinalResult.svelte
+- Collapsed by default
+- Summary line shows key stat (e.g., total tokens + tokens/sec)
+- Expanded content shows all stats in a grid layout
+- Context utilization shown as a progress bar
+- Blue/indigo color scheme (neutral, info-like)
+- Full dark mode support
+
+### Phase 4: Chat Page Integration
+
+**Files:**
+- `src/routes/chat/+page.svelte` — Render `InferenceStatsPanel` after `FinalResult` when stats available
+
+## Acceptance Criteria
+
+- [x] InferenceStats proto message defined and TypeScript types generated
+- [x] InferenceStatsPanel displays all required metrics
+- [x] Panel is collapsible, collapsed by default
+- [x] Context utilization shows visual progress bar
+- [x] Integrates cleanly into chat page below assistant message
+- [x] Dark mode support
+- [x] Build, lint, typecheck pass