Claude Desktop local sessions show a usage-limit ring but no context-window fill indicator

## Summary
In Claude Desktop's local (Claude Code) sessions, the circular indicator beside the composer shows plan usage limits. There is no way to see how full the conversation's context window is while working. The terminal shows context fill in the status line; Desktop shows only the usage ring.

## Why it matters
Without a context-fill signal, long Desktop sessions run to 300k-500k tokens of context before anyone notices, and every reply re-reads all of it. On 18 Sep one Desktop chat reached 467k tokens before it was compacted. Knowing the fill level is what tells a user when to compact.

## Ask
Show context-window fill (percentage or tokens) in Desktop local sessions, alongside or instead of the usage-limit ring, or let the user choose which the ring shows.

## Environment
Claude Desktop 2.110.0, Claude Code 2.1.273, macOS 26.6.

Start by reproducing the behavior in a Claude Desktop local session and compare the Desktop usage-limit ring with the terminal status line's context-fill display. Determine how a context percentage or token count could appear alongside the ring or replace it, then verify that users can see when a session needs compaction.

Written by the indexing model from the issue text.