fix(plan-usage): make the usage chip per-device, not synced

The plan-usage header chip (5h/7d %) was a SYNCED setting, so enabling
it on desktop turned it on for mobile too — even though the user never
enabled it there. Make the chip's DISPLAY purely per-device (default
OFF) like the response viewer / skin, while keeping telemetry COLLECTION
server-side.

Three leak sources fixed:
- server.ts renderIndexHtml force-revealed the chip from the synced
  value (pre-paint), pushing the desktop choice onto every device.
  Removed — the chip now ships hidden and the client reveals it
  per-device via applyHeaderVisibilitySettings.
- settings-ui.js load-merge let the server value win, writing desktop's
  `true` into the (separate) mobile settings blob. showPlanUsageLimits
  is now a displayKey AND is dropped from the server payload on load, so
  a stale server value is never seeded into a device that didn't enable
  it. It's also stripped from the save payload so a mobile "off" can't
  clobber the server.
- Collection was gated on the same synced flag. Decoupled via a new
  `statusLineTelemetry` ACTION field (schema + system-routes): sent on
  ENABLE only and never persisted, so the exporter is injected when a
  device turns the chip on but is never yanked when another device has
  it off (it's shared across sibling sessions). Session-create already
  reads the per-device blob, so that path was already correct.

One-time migration clears a stale synced `true` from the mobile blob so
existing mobile installs default to OFF without a manual toggle.

Verified end-to-end on an isolated server: with showPlanUsageLimits=true
persisted, the rendered HTML ships the chip hidden; a fresh browser
context (mobile case) keeps it hidden while a context that explicitly
enabled it shows it; the PUT accepts statusLineTelemetry and does not
persist it. tsc + frontend-syntax + system-routes/index tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/schemas.ts

@@ -369,6 +369,12 @@ export const SettingsUpdateSchema = z
     showSubagents: z.boolean().optional(),
     showMultiMonitorButton: z.boolean().optional(),
     showPlanUsageLimits: z.boolean().optional(),
+    // Action field (NOT persisted as a setting): when true, (re)injects the
+    // plan-usage statusLine exporter into active Claude sessions so live usage %
+    // starts flowing. Sent on ENABLE only — the chip's DISPLAY is per-device
+    // (client-side), but telemetry COLLECTION is server-side, so the per-device
+    // toggle signals it out-of-band here rather than via showPlanUsageLimits.
+    statusLineTelemetry: z.boolean().optional(),
     // Input
     gestureControlEnabled: z.boolean().optional(),
     // Claude CLI settings