Desktop footer context limit ignores effective ModelConfig (GOOSE_CONTEXT_LIMIT), disagrees with compaction

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**Describe the bug**

Split from #10893. The footer denominator comes from static metadata with a hardcoded 128000 fallback (ui/desktop/src/components/ChatInput.tsx:596-627, :83) and never reads the session's effective ModelConfig, which compaction uses and which honors GOOSE_CONTEXT_LIMIT.

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**To Reproduce**

Set GOOSE_CONTEXT_LIMIT: 1048576, start a session on a model whose static metadata says 128k: footer shows x / 128k while compaction triggers relative to 1M. Provider-agnostic.

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**Expected behavior**

Footer and compaction read the same effective limit. Direction: expose the session's effective context_limit over ACP next to the existing usage updates; ChatInput prefers it, static chain as fallback.

Verification: with an override, footer and compaction agree; without, known models keep canonical limits.