Replace token-usage panel with per-pane context-fill indicator

For a subscription user, lifetime token totals + a $ estimate aren't
actionable; how full each session's context window is right now is. So:

- Removed the UsagePanel, the titlebar 💰 chip, and Ctrl+Shift+U.
- Repurposed the transcript reader (src-tauri/src/usage.rs): get_pane_context
  returns each recent session's CURRENT context occupancy = the last
  assistant turn's input + cache_read + cache_creation tokens (the prompt
  size), instead of lifetime sums. Same UNC/$HOME/cache/recency machinery.
- src/lib/usage.ts now holds context helpers (window inference 200k vs 1M by
  whether occupancy already exceeds 200k, % , green→amber→red ramp, label).
- App polls get_pane_context (15s, visibility-gated) into a cwd→context map
  exposed via orchestration; each LeafPane looks itself up by leaf.cwd and
  renders a slim fill bar + % in its header (hidden for non-claude/unmatched
  panes).

Also fixes the narrow-pane toolbar: a ResizeObserver sets leaf--narrow /
leaf--xnarrow width tiers; the label shrinks first, split buttons / status /
secondary chips drop out by tier, and the close × + context indicator stay
pinned right and visible down to the 180px min width.

tsc clean (apart from the not-yet-installed xterm addons). Rust builds on
the Windows host; needs runtime verification.

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

src-tauri/src/lib.rs

@@ -110,7 +110,7 @@ pub fn run() {
             commands::mcp_policy_load,
             commands::mcp_policy_save,
             commands::mcp_hard_deny_labels,
-            usage::get_claude_usage,
+            usage::get_pane_context,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");