feat: consume context window + percentage-based compact handoff

1. Real context window: GET /models now returns modelInfo[model].contextWindow.
   The Composer uses this instead of the hardcoded MAX_CONTEXT = 1,000,000.
   Falls back to 1M when modelInfo is absent or the model has no contextWindow.

2. Percentage-based auto-compact: the compact-threshold endpoint is renamed
   to compact-percent. The CompactionView now shows a percent input (0-100,
   default 85, 0 = manual) instead of a token count input. Types renamed:
   CompactThresholdResponse → CompactPercentResponse,
   SetCompactThresholdRequest → SetCompactPercentRequest.
   Note: the field name in the backend types is still 'threshold' (not
   'percent') — the FE maps between them.

Re-mirrored .dispatch/transport-contract.reference.md.

686 tests green. 0 svelte-check errors + warnings.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/features/chat/index.ts b/src/features/chat/index.ts
index 9c65cd4..1596c53 100644
--- a/src/features/chat/index.ts
+++ b/src/features/chat/index.ts
@@ -17,7 +17,7 @@ export {
 export type { ChatStore, ChatStoreDependencies } from "./store.svelte";
 export { createChatStore } from "./store.svelte";
 export { default as ChatView } from "./ui/ChatView.svelte";
-export type { CompactNowResult, SaveCompactThresholdResult } from "./ui/CompactionView.svelte";
+export type { CompactNowResult, SaveCompactPercentResult } from "./ui/CompactionView.svelte";
 export { default as CompactionView } from "./ui/CompactionView.svelte";
 export { default as Composer } from "./ui/Composer.svelte";
 export { default as ModelSelector } from "./ui/ModelSelector.svelte";