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/pricing.ts

@@ -0,0 +1,69 @@
+/**
+ * Per-model token pricing for BYOK cost estimation.
+ *
+ * Values in USD per 1M tokens (as published by providers as of
+ * 2026-04). Update manually when providers change pricing.
+ *
+ * Only includes models Mana exposes in the BYOK provider adapters.
+ */
+
+export interface ModelPricing {
+	/** USD per 1 million input tokens */
+	inputPerMillion: number;
+	/** USD per 1 million output tokens */
+	outputPerMillion: number;
+}
+
+export const MODEL_PRICING: Record<string, ModelPricing> = {
+	// ── OpenAI ──────────────────────────────────────────
+	'gpt-5': { inputPerMillion: 15, outputPerMillion: 60 },
+	'gpt-5-mini': { inputPerMillion: 3, outputPerMillion: 12 },
+	'gpt-4o': { inputPerMillion: 5, outputPerMillion: 20 },
+	'gpt-4o-mini': { inputPerMillion: 0.3, outputPerMillion: 1.2 },
+	'gpt-4-turbo': { inputPerMillion: 10, outputPerMillion: 30 },
+	o1: { inputPerMillion: 15, outputPerMillion: 60 },
+	'o1-mini': { inputPerMillion: 3, outputPerMillion: 12 },
+
+	// ── Anthropic ───────────────────────────────────────
+	'claude-opus-4-6': { inputPerMillion: 15, outputPerMillion: 75 },
+	'claude-opus-4-5': { inputPerMillion: 15, outputPerMillion: 75 },
+	'claude-sonnet-4-6': { inputPerMillion: 3, outputPerMillion: 15 },
+	'claude-sonnet-4-5': { inputPerMillion: 3, outputPerMillion: 15 },
+	'claude-haiku-4-5': { inputPerMillion: 0.8, outputPerMillion: 4 },
+
+	// ── Google Gemini ───────────────────────────────────
+	'gemini-2.5-pro': { inputPerMillion: 1.25, outputPerMillion: 5 },
+	'gemini-2.5-flash': { inputPerMillion: 0.15, outputPerMillion: 0.6 },
+	'gemini-2.5-flash-lite': { inputPerMillion: 0.075, outputPerMillion: 0.3 },
+	'gemini-2.0-flash': { inputPerMillion: 0.1, outputPerMillion: 0.4 },
+
+	// ── Mistral ─────────────────────────────────────────
+	'mistral-large-latest': { inputPerMillion: 2, outputPerMillion: 6 },
+	'mistral-medium-latest': { inputPerMillion: 2.7, outputPerMillion: 8.1 },
+	'mistral-small-latest': { inputPerMillion: 0.2, outputPerMillion: 0.6 },
+	'open-mistral-nemo': { inputPerMillion: 0.15, outputPerMillion: 0.15 },
+	'codestral-latest': { inputPerMillion: 0.3, outputPerMillion: 0.9 },
+};
+
+/** USD cost for a given call. Returns 0 if model isn't in the table. */
+export function estimateCost(
+	model: string,
+	promptTokens: number,
+	completionTokens: number
+): number {
+	const p = MODEL_PRICING[model];
+	if (!p) return 0;
+	return (
+		(promptTokens / 1_000_000) * p.inputPerMillion +
+		(completionTokens / 1_000_000) * p.outputPerMillion
+	);
+}
+
+/** Format USD value with at most 4 decimals (for small per-call amounts). */
+export function formatCost(usd: number): string {
+	if (usd === 0) return '—';
+	if (usd < 0.0001) return '< $0.0001';
+	if (usd < 0.01) return `$${usd.toFixed(4)}`;
+	if (usd < 1) return `$${usd.toFixed(3)}`;
+	return `$${usd.toFixed(2)}`;
+}