fix(desktop): include completion tokens in context gauge total (#5283)

[ttmouse]

Summary

Fixes #5283 by making ContextSnapshot() report the current turn total used tokens as promptTokens + completionTokens, so the Desktop context-window gauge and breakdown no longer scale the prompt segment down when completion/reasoning tokens are present.

Changes

• Update internal/control.Controller.ContextSnapshot() from prompt-only usage to total prompt + completion usage.
• Add a controller regression test for the reported 6840 prompt + 48 completion/reasoning = 6888 used case.
• Add a frontend context-panel regression test to ensure prompt tokens are not scaled down for the [Bug]: GUI界面的上下文窗口数值似乎计算小错误 #5283 token breakdown.

Verification

• go test ./internal/control passed
• go test ./internal/control ./internal/cli ./internal/serve passed
• pnpm --dir desktop/frontend exec tsx src/__tests__/context-panel-breakdown.test.ts passed, 19/19
• git diff --check passed

Scope

• 3 files changed: internal/control/controller.go, internal/control/controller_test.go, and desktop/frontend/src/__tests__/context-panel-breakdown.test.ts.

Cache Impact

• No provider request serialization, system prompt, tool schema, memory prefix, or compaction behavior changes.

Cache Guard

• Not applicable; this PR changes controller/UI telemetry reporting and regression tests only.