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, 51 insertions, 50 deletions

diff --git a/src/features/chat/ui/CompactionView.svelte b/src/features/chat/ui/CompactionView.svelte
index ce2a0a0..7bec984 100644
--- a/src/features/chat/ui/CompactionView.svelte
+++ b/src/features/chat/ui/CompactionView.svelte
@@ -3,54 +3,54 @@
 		| { readonly ok: true; readonly messagesSummarized: number; readonly messagesKept: number }
 		| { readonly ok: false; readonly error: string };
 
-	export type SaveCompactThresholdResult =
-		| { readonly ok: true; readonly threshold: number }
+	export type SaveCompactPercentResult =
+		| { readonly ok: true; readonly percent: number }
 		| { readonly ok: false; readonly error: string };
 
 	let {
-		threshold,
+		percent,
 		canCompact,
 		compactNow,
-		saveThreshold,
+		savePercent,
 	}: {
-		/** The conversation's auto-compact threshold, or null when not yet fetched. 0 = disabled. */
-		threshold: number | null;
+		/** The conversation's auto-compact percent (0-100), or null when not yet fetched. 0 = disabled. */
+		percent: number | null;
 		/** Whether a real conversation is focused (a draft has nothing to compact). */
 		canCompact: boolean;
 		compactNow: () => Promise<CompactNowResult | null>;
-		saveThreshold: (threshold: number) => Promise<SaveCompactThresholdResult | null>;
+		savePercent: (percent: number) => Promise<SaveCompactPercentResult | null>;
 	} = $props();
 
-	const DEFAULT_THRESHOLD = 350000;
+	const DEFAULT_PERCENT = 85;
 
 	let compacting = $state(false);
 	let compactError = $state<string | null>(null);
 	let compactResult = $state<{ summarized: number; kept: number } | null>(null);
 
-	let thresholdInput = $state("");
-	let savingThreshold = $state(false);
-	let thresholdError = $state<string | null>(null);
-	let thresholdSaved = $state(false);
+	let percentInput = $state("");
+	let savingPercent = $state(false);
+	let percentError = $state<string | null>(null);
+	let percentSaved = $state(false);
 
 	// Sync the input from the prop when it changes (focus switch / initial load).
-	let lastThreshold = $state<number | null>(null);
+	let lastPercent = $state<number | null>(null);
 	$effect(() => {
-		if (threshold !== lastThreshold) {
-			lastThreshold = threshold;
-			thresholdInput = threshold !== null ? String(threshold) : "";
-			thresholdError = null;
-			thresholdSaved = false;
+		if (percent !== lastPercent) {
+			lastPercent = percent;
+			percentInput = percent !== null ? String(percent) : "";
+			percentError = null;
+			percentSaved = false;
 		}
 	});
 
-	const thresholdLabel = $derived(
-		threshold == null
+	const percentLabel = $derived(
+		percent == null
 			? "Loading…"
-			: threshold === 0
+			: percent === 0
 				? "Disabled (manual only)"
-				: threshold === DEFAULT_THRESHOLD
-					? `${threshold.toLocaleString("en-US")} (default)`
-					: threshold.toLocaleString("en-US"),
+				: percent === DEFAULT_PERCENT
+					? `${percent}% (default)`
+					: `${percent}%`,
 	);
 
 	async function handleCompact() {
@@ -68,22 +68,22 @@
 		}
 	}
 
-	async function handleSaveThreshold() {
-		const value = Number.parseInt(thresholdInput, 10);
-		if (Number.isNaN(value) || value < 0) {
-			thresholdError = "Must be a non-negative number";
+	async function handleSavePercent() {
+		const value = Number.parseInt(percentInput, 10);
+		if (Number.isNaN(value) || value < 0 || value > 100) {
+			percentError = "Must be 0-100";
 			return;
 		}
-		savingThreshold = true;
-		thresholdError = null;
-		thresholdSaved = false;
-		const result = await saveThreshold(value);
-		savingThreshold = false;
+		savingPercent = true;
+		percentError = null;
+		percentSaved = false;
+		const result = await savePercent(value);
+		savingPercent = false;
 		if (result === null) return;
 		if (result.ok) {
-			thresholdSaved = true;
+			percentSaved = true;
 		} else {
-			thresholdError = result.error;
+			percentError = result.error;
 		}
 	}
 </script>
@@ -120,33 +120,34 @@
 		{/if}
 	</section>
 
-	<!-- Auto-compact threshold -->
+	<!-- Auto-compact percent -->
 	<section class="flex flex-col gap-1">
-		<span class="text-xs font-semibold uppercase opacity-60">Auto-compact threshold</span>
+		<span class="text-xs font-semibold uppercase opacity-60">Auto-compact percent</span>
 		<div class="flex items-center gap-2">
 			<input
 				type="number"
-				class="input input-bordered input-sm w-32"
+				class="input input-bordered input-sm w-24"
 				min="0"
-				placeholder={DEFAULT_THRESHOLD.toLocaleString("en-US")}
-				value={thresholdInput}
-				disabled={savingThreshold}
-				onchange={handleSaveThreshold}
-				aria-label="Compact threshold (tokens)"
+				max="100"
+				placeholder={String(DEFAULT_PERCENT)}
+				value={percentInput}
+				disabled={savingPercent}
+				onchange={handleSavePercent}
+				aria-label="Compact percent (0-100)"
 			/>
-			<span class="text-xs opacity-60">tokens</span>
-			{#if savingThreshold}
+			<span class="text-xs opacity-60">%</span>
+			{#if savingPercent}
 				<span class="loading loading-spinner loading-xs"></span>
 			{/if}
 		</div>
 		<p class="text-xs opacity-50">
-			Current: {thresholdLabel}
+			Current: {percentLabel}
 			<br />
-			0 disables auto-compact. Default is {DEFAULT_THRESHOLD.toLocaleString("en-US")}.
+			0 disables auto-compact. Default is {DEFAULT_PERCENT}%.
 		</p>
-		{#if thresholdError}
-			<p class="text-xs text-error">{thresholdError}</p>
-		{:else if thresholdSaved}
+		{#if percentError}
+			<p class="text-xs text-error">{percentError}</p>
+		{:else if percentSaved}
 			<p class="text-xs text-success">Saved.</p>
 		{/if}
 	</section>