feat(popover): migrate the dialog surfaces onto the platform (R8 S4c)

The column filter panel, the nav's account panel and the header's sync read-out are
mixed content with their own tab order rather than lists of choices, so they migrate as
`dialog` popovers — trapping and returning focus, dismissing through the shared layer
stack instead of their own document listeners.

The column filter needed a structural change: its open state lived in the header row
while the panel rendered per column, which cannot work when the platform anchors a panel
to its own trigger. Funnel and panel are now one module-level component (module-level
still matters — declaring it inline once remounted the popover on every keystroke, which
broke the caret and IME composition). That retires three hand-rolled focus chores at
once: capturing the trigger element at open time because a per-column ref nulled out
before focus could return, the effect that stopped Escape stranding focus on <body>, and
the useDismiss registration that had to include the trigger so its own mousedown did not
close-then-reopen the panel.

The filter also stops being one undifferentiated panel: a text filter is a dialog whose
input takes initial focus, while the select and multi filters are real menus with roving
focus — select as radios, multi as checkboxes.

`menuitemcheckbox` is new to the platform, and the two existing multi-select toggle lists
(channel tags, add-to-playlist) move onto it as well: several tags or playlists can be on
at once, so they were never one exclusive choice. Role-only change, no interaction
difference.

Verified in-browser: all three anchor exactly where their hand-rolled placement did,
Escape closes and returns focus to the trigger, the text filter keeps focus and caret
position while typing narrows the table, and the multi filter opens as a checkbox menu
with roving focus.

frontend/src/components/AddToPlaylist.tsx

@@ -108,7 +108,8 @@ export default function AddToPlaylist({
             {lists.map((pl) => (
               <PopoverItem
                 key={pl.id}
-                role="menuitemradio"
+                // A video can sit in several playlists at once — checkboxes, not one choice.
+                role="menuitemcheckbox"
                 checked={pl.has_video}
                 onSelect={() => toggle(pl)}
                 disabled={busy}