[FEATURE] Desktop: option to make the footer usage ring fill with context window usage instead of plan usage

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

In the Claude Desktop app (Code tab), the circular indicator next to the model and effort pill in the composer footer used to fill up as the session's context window filled. After the desktop redesign it became a combined "usage ring": the ring itself now reflects plan usage (session / weekly limits), and the context window fraction is only visible after clicking the ring to open the popover.

For long agentic sessions the number I actually need to watch at a glance is context, not plan usage. Context is what decides when compaction will hit and when I should wrap up or `/compact` deliberately. Plan usage changes slowly and is shared across every surface, so it is much less useful as an always-visible fill. Right now I have to click the ring (or run `/context`) every time I want to know how close I am to the limit, which is the opposite of "at a glance".

The docs describe the current behavior here: https://code.claude.com/docs/en/desktop#check-usage ("Click the usage ring next to the model picker to see your current context window usage and your plan usage for the period.") There is no setting that controls what the ring's fill represents.

Related earlier threads asking for a persistent context indicator on desktop: #33257, #45079, #20041.

### Proposed Solution

Add a user-level setting (Desktop settings, or a `settings.json` key) that chooses what the footer ring fills with:

- `context` (per-session context window usage)
- `plan` (current behavior: session / weekly plan usage)
- `both` (for example an outer ring for context and an inner ring for plan usage, or the ring shows context and a small dot/badge changes color as plan usage nears its limit)

The click popover can keep showing both numbers regardless of the setting. Ideally hovering the ring shows the context fraction as a tooltip (e.g. `132.4k / 200.0k (66%)`) without needing a click.

### Alternative Solutions

- Clicking the ring every time to read the context number (works, but not at a glance).
- Typing `/context` in the composer (heavier, adds output to the transcript).
- The terminal CLI shows context in the status line / footer, but that does not help inside the Desktop app.

### Priority

High - Significant impact on productivity

### Feature Category

Configuration and settings

### Use Case Example

1. I run multi-hour sessions in the Desktop app (large refactors, data reconciliation, multi-file builds).
2. I glance at the footer ring to decide whether to start one more big task or stop and `/compact` first.
3. Today the ring tells me plan usage, so it is nearly full or nearly empty for reasons unrelated to this session, and I get surprised by auto-compaction.
4. With the ring set to `context`, one glance answers the question and I can time compaction myself.

### Additional Context

- Platform: Windows 11 Pro, Claude Desktop app, Code tab.
- Model: Fable 5.1 (the ring sits next to the "high" effort pill).
- The usage ring is described in the desktop redesign post: https://claude.com/blog/claude-code-desktop-redesign ("A new usage button shows both your context window and session usage at a glance.")

Start by locating the Claude Desktop app's Code tab footer usage ring and the Desktop settings or settings.json handling mentioned in the request. Review how the ring currently exposes plan and context usage; done means a user-level choice can select context, plan, or both while the click popover still shows both values.

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