feat(chat): add "Off" option to Reasoning Effort dropdown (thinking on/off)

Add an "Off" choice as the FIRST option in the per-conversation Reasoning
Effort selector. Selecting it disables extended thinking ENTIRELY for the
conversation's turns — NOT "set the effort to its lowest level". The two axes
are kept SEPARATE on the wire (per the umans API model, where thinking-off is
`reasoning_effort: "none"`, distinct from the effort level):

- `reasoningEffort` is the thinking-DEPTH ladder (low→max) — UNCHANGED.
- `thinking` is the on/off switch — a NEW per-conversation boolean.

Turning thinking off PRESERVES the persisted effort level, so an off→on toggle
restores the previously-chosen depth. The selector CONFLATES the two axes into
one <select> (UX); the wire does not.

Pure logic (reasoning-effort.ts): ThinkingSelection = "off" | ReasoningEffort,
selectionOptions() (Off first + the ladder, default marked), isThinkingSelection,
effectiveSelection(persistedEffort, persistedThinking), + FE-local PROPOSED wire
types (ThinkingResponse, SetThinkingRequest) + SaveThinkingSelection port. The
existing effortOptions()/isReasoningEffort()/effectiveEffort() are UNCHANGED — the
heartbeat feature reuses them (its own dropdown is unaffected).

Store: `thinking` state (boolean|null, null⇒ON default), refreshThinking() (GET,
alongside refreshReasoningEffort() on every focus/draft/switch), setThinking()
(PUT). App.svelte's saveThinkingSelection adapter fans one selection out: "off"
→ PUT /thinking {thinking:false} (effort untouched); a level → ensure thinking ON
(if off) then PUT /reasoning-effort {level}.

Backend contract gap (CR-14, backend-handoff.md §2l): the backend has NO
thinking-off mechanism today (umans mapReasoningEffort can't emit "none"). The FE
is built against the PROPOSED additive contract (GET/PUT /conversations/:id/thinking
+ ThinkingResponse/SetThinkingRequest; umans maps thinking===false →
reasoning_effort:"none"). Until shipped, GET /thinking 404s and refreshThinking()
leaves `thinking` null⇒ON — the selector gracefully shows the effort level, no crash.

Verification: typecheck 0/0, 1128 tests green (run TWICE — touches the shared fetch
fake), biome clean, build OK. 9 files (8 source + backend-handoff.md).

1 files changed, 27 insertions, 16 deletions

diff --git a/src/features/chat/ui/ReasoningEffortSelector.svelte b/src/features/chat/ui/ReasoningEffortSelector.svelte
index d982905..6858779 100644
--- a/src/features/chat/ui/ReasoningEffortSelector.svelte
+++ b/src/features/chat/ui/ReasoningEffortSelector.svelte
@@ -1,34 +1,45 @@
 <script lang="ts">
   import type { ReasoningEffort } from "@dispatch/transport-contract";
   import {
-    effectiveEffort,
-    effortOptions,
-    isReasoningEffort,
-    type SaveReasoningEffort,
+    effectiveSelection,
+    isThinkingSelection,
+    selectionOptions,
+    type SaveThinkingSelection,
+    type ThinkingSelection,
   } from "../reasoning-effort";
 
   let {
-    persisted,
+    persistedEffort,
+    persistedThinking,
     save,
   }: {
-    /** The conversation's persisted level, or null when never set (default applies). */
-    persisted: ReasoningEffort | null;
-    save: SaveReasoningEffort;
+    /** The conversation's persisted effort level, or null when never set (default applies). */
+    persistedEffort: ReasoningEffort | null;
+    /**
+     * The conversation's persisted thinking flag, or null when never set
+     * (thinking ON — the default). `false` ⇒ thinking disabled (the separate
+     * "off" axis); the effort level is preserved across an off→on toggle.
+     */
+    persistedThinking: boolean | null;
+    /** Persist a thinking selection (off or a level). */
+    save: SaveThinkingSelection;
   } = $props();
 
-  const options = effortOptions();
+  const options = selectionOptions();
 
-  // 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);
+  // The user's in-flight choice; null = mirror the (async-loaded) persisted
+  // selection. Re-mounted per conversation, so there is no cross-tab bleed.
+  let chosen = $state<ThinkingSelection | null>(null);
   let saving = $state(false);
   let error = $state<string | null>(null);
   let justSaved = $state(false);
 
-  const selected = $derived(chosen ?? effectiveEffort(persisted));
+  const selected = $derived(
+    chosen ?? effectiveSelection(persistedEffort, persistedThinking),
+  );
 
   async function handleChange(value: string) {
-    if (!isReasoningEffort(value) || saving) return;
+    if (!isThinkingSelection(value) || saving) return;
     chosen = value;
     saving = true;
     error = null;
@@ -40,7 +51,7 @@
       justSaved = true;
     } else {
       error = result.error;
-      chosen = null; // revert to the persisted value
+      chosen = null; // revert to the persisted selection
     }
   }
 </script>
@@ -69,7 +80,7 @@
     <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.
+      How long the model thinks before answering. “Off” disables thinking entirely. Changing it can re-prefill the prompt cache once.
     </p>
   {/if}
 </div>