remove(llm): retire think_enabled setting entirely

Two-in-one cleanup motivated by the chat hang in dev 2026-05-22.

The crash root cause from the guarded-task traceback:

    UnboundLocalError: cannot access local variable 'get_setting'
    where it is not associated with a value
      File generation_task.py:257, in run_generation
        think = (await get_setting(user_id, 'think_enabled', 'false'))...

generation_task.py imports get_setting at module top, but a later
'if voice_mode: from ... import get_setting' block scopes it as a
function-local. When voice_mode=False the local import never runs,
but Python had already flagged get_setting as local for the entire
body — the think_enabled read at line 257 hit UnboundLocalError.

The line itself was dead-weight anyway. With the conversation+curator
architecture: chat ships tools=[] (think on a no-tools pass is pure
latency cost; nothing for the model to reason ABOUT in tool-call
terms), and the curator hardcodes think=False already. The user
setting was a holdover from before the architecture pivot. Removing
it entirely is cleaner than fixing the scoping bug to preserve a
toggle nobody should be using:

- generation_task.py: think hardcoded False. Removed the get_setting
  call (which fixes the UnboundLocalError as a side effect).
- SettingsView.vue: dropped the Enable model thinking checkbox, the
  thinkEnabled / savingThinkEnabled refs, the saveThinkEnabled
  function, and the think_enabled load step.
- Migration 0050: DELETE FROM settings WHERE key='think_enabled'
  to clean up any stored rows.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/views/SettingsView.vue

@@ -20,8 +20,6 @@ const savingTimezone = ref(false);
 const timezoneSaved = ref(false);
 const autoConsolidateTasks = ref(true);
 const savingAutoConsolidate = ref(false);
-const thinkEnabled = ref(false);
-const savingThinkEnabled = ref(false);
 
 async function saveAutoConsolidate() {
   savingAutoConsolidate.value = true;
@@ -36,18 +34,10 @@ async function saveAutoConsolidate() {
   }
 }
 
-async function saveThinkEnabled() {
-  savingThinkEnabled.value = true;
-  try {
-    await apiPut('/api/settings', {
-      think_enabled: thinkEnabled.value ? "true" : "false",
-    });
-  } catch {
-    toastStore.show('Failed to save setting', 'error');
-  } finally {
-    savingThinkEnabled.value = false;
-  }
-}
+// think_enabled setting removed 2026-05-23. The chat+curator architecture
+// has tools=[] on the chat model; think on a no-tools conversational pass
+// is pure latency cost. See generation_task.py:run_generation comment for
+// the full reasoning.
 
 function detectTimezone() {
   userTimezone.value = Intl.DateTimeFormat().resolvedOptions().timeZone;
@@ -826,9 +816,6 @@ onMounted(async () => {
   userTimezone.value = allSettings.user_timezone ?? "";
   // Default true if unset; explicit "false" disables auto-consolidation.
   autoConsolidateTasks.value = (allSettings.auto_consolidate_tasks ?? "true") !== "false";
-  // Default false: think mode is opt-in. Bench showed it costs 1-2 min/turn
-  // for unclear benefit; qwen3 users can re-enable if tool calls regress.
-  thinkEnabled.value = (allSettings.think_enabled ?? "false") === "true";
   chatRetentionDays.value = allSettings.chat_retention_days !== undefined
     ? Number(allSettings.chat_retention_days)
     : 90;
@@ -1569,22 +1556,6 @@ function formatUserDate(iso: string): string {
               </span>
             </p>
           </div>
-          <div class="field">
-            <label class="checkbox-label">
-              <input
-                type="checkbox"
-                v-model="thinkEnabled"
-                :disabled="savingThinkEnabled"
-                @change="saveThinkEnabled"
-              />
-              Enable model thinking
-            </label>
-            <p class="field-hint">
-              When on, the chat model runs in reasoning mode before responding.
-              Default off — bench data showed thinking adds 1–2 minutes per turn for unclear quality benefit.
-              Turn on for qwen3-family models if tool-call reliability drops without it.
-            </p>
-          </div>
         </div>
         <div class="actions">
           <button class="btn-save" @click="saveAssistant" :disabled="saving">