refactor(chips): share the chip-overflow measure core between the two lanes (S3)

ActiveFilterChips (feed applied-filter row) and ChannelTagsControl both
hand-rolled the same single-line chip-fit measurer: a hidden mirror + a
layout-effect + ResizeObserver reading chip right edges, the real rem gap,
the pill width and the lane width. The policy half was already extracted
(decideChipLayout); the DOM-measurement half was still duplicated.

Promote that half to useChipOverflowFit(rowRef, chipsRef, pillRef, deps,
decide, initial): it owns the effect + observer + initial measure and reads
the common geometry, then hands it to each caller's own policy. The feed
row layers its leading label + trailing "Clear all" reservation on top and
keeps decideChipLayout; the tag cell's chips-only policy is extracted as
fitChipsWithPill (pure, unit-tested) alongside it. Add a jsdom test for the
hook (stubbed offsets, re-measure on deps). Behaviour unchanged — verified
in the browser: feed all/count modes + popover, tag "+N" fit + hover reveal.

frontend/src/lib/chipOverflow.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { decideChipLayout, type ChipFitInput } from "./chipOverflow";
+import { decideChipLayout, fitChipsWithPill, type ChipFitInput } from "./chipOverflow";
 
 // Build measurer-style right edges for `n` chips of a fixed width, starting after a label, with a gap.
 function rights(labelRight: number, widths: number[], gap: number): number[] {
@@ -91,3 +91,29 @@ describe("decideChipLayout", () => {
     expect(decideChipLayout(input)).toEqual({ mode: "count", visible: 0 });
   });
 });
+
+describe("fitChipsWithPill", () => {
+  // Three chips of width 50, gap 6, starting at 0 -> right edges 50, 106, 162.
+  const chipRights = rights(0, [50, 50, 50], 6);
+
+  it("shows all chips (no pill) when the last chip's right edge fits — the whole-set fast path", () => {
+    expect(fitChipsWithPill({ chipRights, pillW: 40, gap: 6, avail: 200 })).toBe(3);
+  });
+
+  it("returns 0 for an empty chip set", () => {
+    expect(fitChipsWithPill({ chipRights: [], pillW: 40, gap: 6, avail: 200 })).toBe(0);
+  });
+
+  it("reserves the pill width for all but the last chip when overflowing", () => {
+    // avail 120: whole set (162) doesn't fit. chip0 right 50 <= 120-(40+6)=74 ok; chip1 right 106 > 74
+    // -> dropped at i=1 -> visible 1.
+    expect(fitChipsWithPill({ chipRights, pillW: 40, gap: 6, avail: 120 })).toBe(1);
+  });
+
+  it("keeps the whole set via the fast path even when a reserve-for-all loop would drop chips", () => {
+    // Right edges 56, 112, 118; avail 120: the whole set fits (118 <= 120). A naive loop that reserves
+    // pill+gap (46) for every non-last chip would drop at chip1 (112 > 120-46=74) and show "+2" beside
+    // empty space — the fast path returns all 3 instead.
+    expect(fitChipsWithPill({ chipRights: [56, 112, 118], pillW: 40, gap: 6, avail: 120 })).toBe(3);
+  });
+});