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/AnchoredPopover.tsx

@@ -197,10 +197,11 @@ interface PopoverListContextValue {
 }
 const PopoverListContext = createContext<PopoverListContextValue | null>(null);
 
-type ItemRole = "menuitem" | "menuitemradio" | "option";
+type ItemRole = "menuitem" | "menuitemradio" | "menuitemcheckbox" | "option";
 
 /** One keyboard-navigable option inside a menu/listbox `PopoverSurface`. Self-registers for roving
- *  focus + typeahead and applies the correct ARIA. Use `menuitemradio`+`checked` for a toggle,
+ *  focus + typeahead and applies the correct ARIA. Use `menuitemradio`+`checked` when the items are
+ *  one exclusive choice, `menuitemcheckbox`+`checked` when several can be on at once,
  *  `option`+`selected` for a listbox choice, plain `menuitem` for an action.
  *
  *  The active (roving) item carries `data-active` — style the highlight off THAT, not `:focus-visible`:
@@ -237,7 +238,7 @@ export function PopoverItem({
   const active = ctx?.activeIndex === index;
 
   const ariaProps =
-    role === "menuitemradio"
+    role === "menuitemradio" || role === "menuitemcheckbox"
       ? { role, "aria-checked": !!checked }
       : role === "option"
         ? { role, "aria-selected": !!selected }