feat(api,web): budget headroom endpoint + UI chips (Phase 6)

Adds GET /api/budget returning per-provider rolling-window headroom (empty when
budget gating is unconfigured), wired from serve.go via SetBudget. The web UI
polls it and renders a chip per limited provider in the header, flagging any
under 20% remaining. New pure JS helpers formatBudgetHeadroom/
renderBudgetHeadroom are unit-tested; the endpoint is covered by Go handler
tests (empty/reports/500). UI render not browser-verified in this environment.

Completes Phase 6: spend accounting + dispatcher gating + observability surface.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

1 files changed, 21 insertions, 0 deletions

diff --git a/web/style.css b/web/style.css
index f4a9d91..b6f0484 100644
--- a/web/style.css
+++ b/web/style.css
@@ -2034,3 +2034,24 @@ dialog label select:focus {
   font-size: 0.85rem;
   padding: 6px 10px;
 }
+
+/* Budget headroom chips */
+.budget-bar {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 6px;
+  align-items: center;
+}
+.budget-chip {
+  font-size: 0.72rem;
+  padding: 2px 8px;
+  border-radius: 999px;
+  background: var(--bg);
+  border: 1px solid var(--border);
+  color: var(--text-muted);
+  white-space: nowrap;
+}
+.budget-chip--low {
+  border-color: var(--state-budget-exceeded);
+  color: var(--state-budget-exceeded);
+}