fix(statusline): GET /api/settings never writes, and a save sends the collection switch only on a flip

Two follow-ups to #361's sticky telemetry switch.

GET /api/settings reconciled an absent showPlanUsageLimits by persisting
true, but readJsonConfig() answers {} for ANY read failure (a parse
error, EACCES, EMFILE, a read landing inside PUT's non-atomic write), not
only ENOENT, and every page load calls this route, so one unlucky read
replaced the whole settings file with a one-key file. The route is a
plain read again and the default moved into the reader:
readPlanUsageTelemetryEnabled() treats an absent key as ON, the same way
readWorkspaceHooksEnabled() does, which is what the desktop chip already
shows for an install that never touched the setting.

saveAppSettings() sent showPlanUsageLimits on every save. The chip
defaults OFF on handhelds, so a phone saving its font size persisted
false and switched collection off for every desktop, whose chip then
went stale with no error anywhere. The key is now stripped like the
other per-device display keys and re-added only when the save FLIPS the
chip relative to what the device had (planUsageCollectionFlip), so an
explicit toggle on any device still writes it in either direction.

Tests pin both: the GET route with a mocked filesystem (absent, missing,
EACCES, garbage, explicit), the reader default, and the flip helper plus
its wiring in saveAppSettings.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/web/public/settings-ui.js

@@ -2315,21 +2315,25 @@ Object.assign(CodemanApp.prototype, {
     // and syncing would leak mobile's hidden-checkbox false onto desktop); it's
     // also absent from SettingsUpdateSchema, which is .strict() — sending it
     // would 400 the whole settings PUT.
-    // showPlanUsageLimits is the ONE exception to "per-device keys never sync":
-    // its DISPLAY stays per-device (loadAppSettingsFromServer only seeds it into
-    // localStorage when a device has no value yet — same as every other display
-    // key), but it ALSO doubles as the server-side plan-usage telemetry
+    // showPlanUsageLimits is per-device for DISPLAY (loadAppSettingsFromServer
+    // only seeds it into localStorage when a device has no value yet, like every
+    // other display key) but ALSO doubles as the server-side plan-usage telemetry
     // COLLECTION switch (readPlanUsageTelemetryEnabled in hooks-config.ts, read
-    // fresh at every claude session create/respawn), so unlike the others it
-    // MUST flow through in `serverSettings` below on every save — including
-    // OFF, which used to be un-sendable under the old one-way "ENABLE only"
-    // action field this replaces.
+    // fresh at every claude session create/respawn). So it is stripped here like
+    // the others and re-added below ONLY when this save FLIPS it on this device
+    // (planUsageCollectionFlip): the chip defaults OFF on handhelds, so sending
+    // it on every save let a phone saving its font size persist `false` and
+    // switch collection off for every desktop, whose chip then went stale with
+    // no error anywhere. An explicit toggle on any device still writes it, in
+    // either direction.
+    const _chipFlip = this.planUsageCollectionFlip(_prev, settings.showPlanUsageLimits);
     const {
       localEchoEnabled: _leo,
       cjkInputEnabled: _cjk,
       extendedKeyboardBar: _ekb,
       skin: _skin,
       language: _language,
+      showPlanUsageLimits: _pul,
       showAttachmentsButton: _ahb,
       showFileViewerButton: _fvb,
       webglRendererEnabled: _wgl,
@@ -2364,6 +2368,7 @@ Object.assign(CodemanApp.prototype, {
     try {
       const res = await this._apiPut('/api/settings', {
         ...serverSettings,
+        ...(_chipFlip !== undefined ? { showPlanUsageLimits: _chipFlip } : {}),
         notificationPreferences: notifPrefsToSave,
         voiceSettings,
       });
@@ -2637,6 +2642,18 @@ Object.assign(CodemanApp.prototype, {
     return s.showPlanUsageLimits ?? this.getDefaultSettings().showPlanUsageLimits ?? true;
   },
 
+  // What a settings save tells the server about plan-usage COLLECTION: the new
+  // chip value when this save FLIPS it relative to what this device resolved
+  // before (stored value, else the per-device default), otherwise undefined,
+  // meaning "say nothing". The server reads an absent key as ON, so a device
+  // that never touched the chip leaves collection alone, and a handheld (chip
+  // default OFF) cannot switch it off for every desktop by saving its font
+  // size. Pure so test/plan-usage-collection-flip.test.ts can drive it.
+  planUsageCollectionFlip(prevSettings, now) {
+    const before = this.planUsageChipEnabled(prevSettings ?? {});
+    return now === before ? undefined : now;
+  },
+
   applyHeaderVisibilitySettings() {
     const settings = this.loadAppSettingsFromStorage();
     const defaults = this.getDefaultSettings();