feat(llm): add BYOK tier + 4 provider adapters (OpenAI, Anthropic, Gemini, Mistral)

Phase 1-3 of BYOK support. Introduces a 5th LLM tier 'byok' that
routes to user-provided API keys via direct browser fetches.

shared-llm additions:
- LlmTier extended with 'byok' (rank 3, between mana-server and cloud)
- ByokBackend: LlmBackend implementation that delegates key lookup
  to an app-provided resolver callback, then dispatches to the right
  provider adapter
- 4 provider adapters:
  - OpenAI (gpt-5, gpt-4o, o1 family)
  - Anthropic (Claude Opus/Sonnet/Haiku 4.6) with CORS header
  - Gemini (2.5 Pro/Flash) — REST API with different message format
  - Mistral — OpenAI-compatible, reuses shared openai-compat adapter
- Pricing table for 20+ models with USD per 1M tokens
- estimateCost() + formatCost() helpers

Keys stay device-local (IndexedDB in next phase). Browser-direct
fetches mean keys never touch Mana's server.

Updates two existing tier maps (memoro DetailView, SourceBadge) to
include the new tier.

Planning doc at docs/architecture/BYOK_PLAN.md.

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

packages/shared-llm/src/orchestrator.ts

@@ -70,6 +70,18 @@ export class LlmOrchestrator {
 		this.settings = settings;
 	}
 
+	/** Register (or replace) a backend at runtime — used by the app
+	 *  to wire up the BYOK backend after initial orchestrator construction,
+	 *  since BYOK needs access to app-side IndexedDB keys. */
+	registerBackend(backend: LlmBackend): void {
+		this.backendsByTier.set(backend.tier, backend);
+	}
+
+	/** Remove a backend (e.g. when the user disables BYOK). */
+	unregisterBackend(tier: LlmTier): void {
+		this.backendsByTier.delete(tier);
+	}
+
 	/** Public read-only view for UI components that want to react to
 	 *  the current settings (e.g. the tier selector). */
 	getSettings(): Readonly<LlmSettings> {