fix(channels): trim fixedLayout column widths to fit narrower viewports (R14 F5)

Shave the manager table's fixed column widths (fixed sum 720->632px) and
convert priority/actions from px to rem so they scale with the text-size
slider like their siblings: tags 12->9rem, actions 104px->5.5rem,
lastUpload 6.5->6rem, stored 5.5->5rem, length 5->4.5rem, priority
56px->3.5rem. Gives the fluid Channel/About columns ~90px more room, so the
table fits without a horizontal scrollbar down to ~700px containers instead
of overflowing at the md/iPad-portrait band.

frontend/src/components/channelColumns.tsx

@@ -183,7 +183,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       key: "priority",
       header: t("channels.cols.priority"),
       align: "center",
-      width: "56px",
+      width: "3.5rem",
       sortable: true,
       hideInCard: true,
       sortValue: (c) => c.priority,
@@ -201,7 +201,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       header: t("channels.cols.actions"),
       align: "left",
       nowrap: true,
-      width: "104px",
+      width: "5.5rem",
       cardLabel: false,
       render: (c) => (
         <ActionsCell
@@ -249,7 +249,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       header: t("channels.cols.stored"),
       align: "right",
       nowrap: true,
-      width: "5.5rem",
+      width: "5rem",
       sortable: true,
       sortValue: (c) => c.stored_videos,
       // The stored count carries the "fully synced" ✓ charm (R14 folded the retired Sync column in).
@@ -260,7 +260,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       key: "lastUpload",
       header: t("channels.cols.lastUpload"),
       nowrap: true,
-      width: "6.5rem",
+      width: "6rem",
       sortable: true,
       sortValue: (c) => (c.last_video_at ? new Date(c.last_video_at).getTime() : 0),
       render: (c) => <span className="text-muted">{relativeTime(c.last_video_at) || "—"}</span>,
@@ -270,7 +270,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       header: t("channels.cols.length"),
       align: "right",
       nowrap: true,
-      width: "5rem",
+      width: "4.5rem",
       sortable: true,
       sortValue: (c) => c.total_duration_seconds,
       render: (c) => (
@@ -285,7 +285,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       cardLabel: false,
       // Bounded width so the cell can't widen the column and reflow the whole table; the control
       // fills it and collapses overflow into "+N" (see ChannelTagsControl).
-      width: "12rem",
+      width: "9rem",
       filter: {
         kind: "multi",
         options: userTags.map((tg) => ({ value: String(tg.id), label: tg.name })),