feat(popover): add the Floating UI anchored-popover platform + migrate TagsCell pilot (R8 S1)

Introduce useAnchoredPopover + PopoverSurface over @floating-ui/react — one primitive
replacing the hand-rolled measured engines: flip() (two-way flip), shift() (width clamp),
hide() (hide-and-follow when the trigger leaves its scroller), autoUpdate (scroll follow),
FloatingPortal (escape ancestor stacking contexts). Supports menu + hover modes; focus
management lands in S2.

Wrap the app and the public watch page in <FloatingTree> so Escape / outside-press dispatch
is topmost-only across nested layers (S3 folds in the Modal + players).

Migrate the richest engine, TagsCell, as the pilot — ~140 lines of bespoke positioning
(coords state, flip layout-effect, scroll rAF loop, useDismiss, Overlay) collapse into one
hook call. Verified in-app: click opens, portals to <body>, positions below the trigger,
role=menu + aria-expanded/haspopup, Escape and outside-click close, no console errors.

frontend/package.json

@@ -22,6 +22,7 @@
     "@dnd-kit/core": "^6.3.1",
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
+    "@floating-ui/react": "^0.27.20",
     "@tanstack/react-query": "^5.51.0",
     "@tanstack/react-virtual": "^3.14.3",
     "clsx": "^2.1.1",