feat(plex): port the saved-views (personal views) UI to Plex (frontend)

Plex now has the same personal-views system as the feed — the rail header icon
opens the quick-pick popover, the rail title opens the manager modal (list /
create / rename / delete / reorder / set-default / apply), and the active view
leads the chip row with a "modified" dot.

One SavedViewsProvider serves both modules: it switches `source` (feed ⟷ plex)
by the active page (both filter providers are global, so it reads whichever the
page owns) and runs the same logic on per-source wiring — the live filters,
apply, a stable signature (feed filtersToParams / plex plexFiltersSignature),
coercion (asFilters / coercePlexFilters), and what to strip on save. Moved below
PlexProvider in the tree so it can read plex state; resets sticky/modal on a
source switch. The shared popover/modal/chip are unchanged; the modal describes
a plex view's captured filters via a pure buildPlexActiveFilters (extracted from
usePlexActiveFilters — no duplication). api.savedViews/createView take a source;
`SavedView.filters` is now an opaque blob. New coercePlexFilters/plexFilters-
Signature (also dedups PlexProvider's collection migration). Empty-state copy
genericised (no longer says "feed"). A Plex view captures the rail filters (not
watch-state/sort/scope).

Verified end-to-end on localdev: create/apply/preview/default/delete on plex,
feed views stay separate + unregressed (402 vitest), no console errors.

frontend/src/main.tsx

@@ -59,10 +59,12 @@ const root =
           <ConfirmProvider>
             <NavigationProvider>
               <FeedFiltersProvider>
-                <SavedViewsProvider>
-                  <PlaylistsProvider>
-                    <PlexProvider>
-                      <PlexViewBridge>
+                <PlaylistsProvider>
+                  <PlexProvider>
+                    <PlexViewBridge>
+                      {/* SavedViewsProvider sits below BOTH filter providers so it can switch its
+                          source (feed ⟷ plex) by the active page and read the right live filters. */}
+                      <SavedViewsProvider>
                         <ChannelsProvider>
                           <FeedViewProvider>
                             <PrefsProvider>
@@ -72,10 +74,10 @@ const root =
                             </PrefsProvider>
                           </FeedViewProvider>
                         </ChannelsProvider>
-                      </PlexViewBridge>
-                    </PlexProvider>
-                  </PlaylistsProvider>
-                </SavedViewsProvider>
+                      </SavedViewsProvider>
+                    </PlexViewBridge>
+                  </PlexProvider>
+                </PlaylistsProvider>
               </FeedFiltersProvider>
             </NavigationProvider>
           </ConfirmProvider>