feat(profile): keep_screen_awake user preference (default on)

New users column surfaced as an On/Off dropdown in the profile form.
The form and API carry "true"/"false" strings with the theme-style
""-guard, so JSON clients that omit the field can't clobber it;
anything else is rejected. GET/PUT /api/auth/profile expose
keep_screen_awake alongside the other profile fields, and the reader
route threads the preference into its template metadata.

Integration tests pin the schema default, both values, omission
semantics, and rejection — dedicated test user, never testuser.

templates/profile_form.templ

@@ -87,6 +87,16 @@ templ ProfileForm(user User, actionURL string, requireCurrentPassword bool, show
 						}
 					</select>
 				</div>
+				<div>
+					<label class="block text-xs font-semibold uppercase tracking-wide mb-1.5" style="color: var(--text-secondary)">Keep Screen Awake</label>
+					<select
+						name="keep_screen_awake"
+						class="input"
+					>
+						<option value="true" selected?={ user.KeepScreenAwake }>On</option>
+						<option value="false" selected?={ !user.KeepScreenAwake }>Off</option>
+					</select>
+				</div>
 				<div>
 					<label class="block text-xs font-semibold uppercase tracking-wide mb-1.5" style="color: var(--text-secondary)">Timezone</label>
 					<select