feat(chat): reasoning-effort selector — sticky per-conversation thinking-depth knob

Consume the backend's reasoning-effort handoff ([email protected] ReasoningEffort +
[email protected] GET/PUT /conversations/:id/reasoning-effort,
ChatRequest.reasoningEffort): a 5-level selector in the sidebar Model view,
under the provider + model dropdowns. null renders as 'high (default)' per
the server-owned resolution chain; PUT on change (effective next turn);
error + revert on 400; per-conversation re-mount incl. drafts (the draft id
survives promotion, so an effort set on a draft applies from turn 1).
Re-mirrored .dispatch references; GLOSSARY 'reasoning effort'; handoff
updated. 616 tests green; live curl probe passed.

1 files changed, 75 insertions, 0 deletions

diff --git a/src/features/chat/ui/ReasoningEffortSelector.svelte b/src/features/chat/ui/ReasoningEffortSelector.svelte
new file mode 100644
index 0000000..8c7b193
--- /dev/null
+++ b/src/features/chat/ui/ReasoningEffortSelector.svelte
@@ -0,0 +1,75 @@
+<script lang="ts">
+	import type { ReasoningEffort } from "@dispatch/transport-contract";
+	import {
+		effectiveEffort,
+		effortOptions,
+		isReasoningEffort,
+		type SaveReasoningEffort,
+	} from "../reasoning-effort";
+
+	let {
+		persisted,
+		save,
+	}: {
+		/** The conversation's persisted level, or null when never set (default applies). */
+		persisted: ReasoningEffort | null;
+		save: SaveReasoningEffort;
+	} = $props();
+
+	const options = effortOptions();
+
+	// The user's in-flight choice; null = mirror the (async-loaded) persisted prop.
+	// Re-mounted per conversation, so there is no cross-tab bleed.
+	let chosen = $state<ReasoningEffort | null>(null);
+	let saving = $state(false);
+	let error = $state<string | null>(null);
+	let justSaved = $state(false);
+
+	const selected = $derived(chosen ?? effectiveEffort(persisted));
+
+	async function handleChange(value: string) {
+		if (!isReasoningEffort(value) || saving) return;
+		chosen = value;
+		saving = true;
+		error = null;
+		justSaved = false;
+		const result = await save(value);
+		saving = false;
+		if (result === null) return;
+		if (result.ok) {
+			justSaved = true;
+		} else {
+			error = result.error;
+			chosen = null; // revert to the persisted value
+		}
+	}
+</script>
+
+<div class="flex flex-col gap-1">
+	<span class="text-xs font-semibold uppercase opacity-60">Reasoning effort</span>
+	<div class="flex items-center gap-2">
+		<select
+			class="select select-sm w-full"
+			value={selected}
+			disabled={saving}
+			onchange={(e) => handleChange(e.currentTarget.value)}
+			aria-label="Reasoning effort"
+		>
+			{#each options as option (option.value)}
+				<option value={option.value}>{option.label}</option>
+			{/each}
+		</select>
+		{#if saving}
+			<span class="loading loading-spinner loading-xs" aria-label="Saving reasoning effort"></span>
+		{/if}
+	</div>
+	{#if error}
+		<p class="text-xs text-error">{error}</p>
+	{:else if justSaved}
+		<p class="text-xs text-success">Saved — applies from the next turn.</p>
+	{:else}
+		<p class="text-xs opacity-50">
+			How long the model thinks before answering. Changing it can re-prefill the prompt cache once.
+		</p>
+	{/if}
+</div>