feat(chat): paste-to-attach images/PDFs with model capability check

Add multimodal image/PDF input to the chat box via clipboard paste, gated by a
graceful per-model capability check.

UX: a pasted image/PDF inserts an inline token (【image:…】 / 【pdf:…】) into the
draft, so attachments have ORDER relative to typed text and can be referenced
positionally. The token is the only handle — deleting it (atomic Backspace/
Delete, or selection overlap) detaches the file; an input-reconciliation safety
net detaches any attachment whose token is no longer intact. No preview strip.

Capability check: resolveModelCapabilities reads models.dev modalities.input
(new GET /models/capabilities, mirrors /context-limit). The input blocks Send
(no tokens spent) only on a definitive 'no'; unknown capability (catalog offline
/ unmapped provider) stays permissive. Attachments require a fresh turn — Send is
blocked while generating and /chat rejects content mid-turn (409).

Attachments are EPHEMERAL: forwarded to the model for the turn via ordered AI SDK
ImagePart/FilePart content, but never persisted (history keeps the text with
[image]/[pdf] markers). Text-only turns serialize byte-identically to before.

Limits (Anthropic-aligned, enforced at paste + re-validated server-side):
PNG/JPEG/WebP/GIF/PDF; image ≤5MB, PDF ≤32MB, ≤20 attachments, ≤32MB total.

core: UserContentPart types, models/attachments validator, capability resolver,
agent.run+toModelMessages thread ordered content. api: /chat content validation +
passthrough. frontend: attachment-tokens helper, ChatInput paste/token/gating,
per-tab staged attachments, App.svelte capability fetch. +44 tests.

1 files changed, 18 insertions, 0 deletions

diff --git a/packages/api/src/routes/models.ts b/packages/api/src/routes/models.ts
index eeb6029..a1700b1 100644
--- a/packages/api/src/routes/models.ts
+++ b/packages/api/src/routes/models.ts
@@ -20,6 +20,7 @@ import {
 	refreshAccountCredentialsAsync,
 	resolveApiKey,
 	resolveContextLimit,
+	resolveModelCapabilities,
 	selectHaikuModel,
 	setApiKey,
 	validateAccountCredentials,
@@ -180,6 +181,23 @@ modelsRoutes.get("/context-limit", async (c) => {
 	return c.json({ contextLimit });
 });
 
+// Resolve a model's image / PDF INPUT capabilities from the models.dev catalog.
+// Returns `{ capabilities: { image, pdf } | null }`. `null` means UNKNOWN — the
+// provider is unmapped, the model is absent, the catalog predates the
+// `modalities` field, or the catalog is offline. The frontend treats `null` as
+// "can't verify" (optimistic allow) and a definitive `{ image: false }` as a
+// hard block (no tokens spent).
+modelsRoutes.get("/capabilities", async (c) => {
+	const provider = c.req.query("provider");
+	const modelId = c.req.query("modelId");
+	if (!provider || !modelId) {
+		return c.json({ error: "provider and modelId query parameters are required" }, 400);
+	}
+
+	const capabilities = await resolveModelCapabilities(provider, modelId);
+	return c.json({ capabilities });
+});
+
 // List available Claude accounts with validated credentials
 modelsRoutes.get("/claude-accounts", async (c) => {
 	const candidates = resolveClaudeAccounts();