feat(channels): horizontal −/＋ priority stepper with pop-free inline edit (S5)

Redesign the shared PriorityStepper (channel manager cell + detail header)
from a value-plus-stacked-arrows control into a horizontal [−] value [＋]
segmented stepper. The value now sits centred between the buttons and opens
an inline editor in a FIXED-width slot, so click-to-type no longer "pops"
the control wider (the old absolutely-positioned overlay grew w-6 → w-14).

One `size` prop drives the two surfaces: `sm` draws its own bordered pill
for the standalone table cell; `md` drops the outer border and keeps only
the segment dividers, blending into the detail header's lozenge instead of
double-bordering it. Hold-to-repeat, negatives, and the debounced-flush
persistence hook are unchanged.

The manager priority column widens 3.5rem → 5.5rem to fit the horizontal
control under fixed table layout.

Tests: 7 new DOM tests for the stepper interaction contract.

frontend/src/components/ChannelPage.tsx

@@ -418,6 +418,7 @@ export default function ChannelPage({
                     priority={ch.priority}
                     onPriority={(d) => bumpPriority(channelId, d)}
                     onSet={(v) => setPriority(channelId, v)}
+                    size="md"
                   />
                 </div>
               )}