[ziyadakl]

Summary

Brings back the composer's context window meter (the small pie chart left of the send button; click it to compact). Upstream removed it in pingdotgg#33131. It is always shown once the provider reports usage; the removed Labs toggle is not restored.

 ChatView
+  compactDisabledReason (computed again)
   <ChatComposer>
+    contextWindowEntries / LiveUsage / ProviderThread
+    compactDisabledReason
     <ComposerFooterPrimaryActions>
+      <ContextWindowMeter>   (apps/web/src/components/chat/ContextWindowMeter.tsx, fork feature)
+        click -> onCompactContext (existing compactThreadContextStable)
       <ComposerPrimaryActions>

Left alone: the Thread details card (Settings > Appearance > Thread details > Context window), plan mode, the legacy sidebar, and everything else pingdotgg#33131 removed.

Changes from the pre-removal meter:

• Meter popup uses the current formatContextWindowCompactionMessage(threshold) (no model name), so the model-name helper and the loading placeholder are not restored.
• formatContextWindowCost now lives inside the meter file; providerSupportsManualCompaction is exported again from compaction.logic.ts.

Evidence

• Before: composer footer has no meter; only /compact compacts.
  After: meter renders when activeContextWindow exists; click calls the same compact action as /compact.
• effect-tsgo tsrs --noEmit in apps/web: 0 errors.
• vp test run --project unit compaction.logic.test.ts contextWindow.test.ts: 2 files, 13 tests passed.
• vp fmt --check and vp lint on the 5 touched files: clean.
• Not run: browser rendering check (no display on the server).

Merge Danger

Door: two-way

Revert the commit to remove the meter.

Blast Radius: personal

Fork only. Possible future upstream-sync conflicts in ChatComposer.tsx, ChatView.tsx and ComposerFooterControls.tsx; the diff is about 50 lines plus one new file.

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