Add GET_CONTENT picker support, fix account/login bugs, restyle selection UI

- Let Noo be picked as a source (Files + Photos) by other apps' native
  file/photo pickers via Android's GET_CONTENT intent, mirroring the
  existing "Share to Noo" plumbing: MainActivity.kt/PickIntentService/
  PickRequest, ServerProvider.pickRequest/confirmPick/cancelPick, and
  pick-mode tap handling in FilesView/PhotosView. While picking, the
  bottom nav and MoreTabsButton are restricted to Files/Photos only.
- Fix two account bugs: testConnection() failures no longer delete a
  still-valid stored app password (only an actual 401 does), and
  switchAccount() now retries a nominally-active-but-logged-out account
  instead of no-op'ing - both were silently locking users out of "Continue
  as" with no way back in except removing and re-adding the account
  (now exposed via login_view.dart's inline remove button).
- Fix the login WebView: clear cookies before every load so "Add Account"
  can't silently reuse an existing session, and push it as a
  non-animated route to avoid a known WebView-as-PlatformView black-screen-
  on-pop issue under Impeller.
- Redesign the Files/Photos selection toolbar to take over the top app bar
  entirely (SyncedHeaderScaffold.selectionBar) instead of sitting as a
  second bar under the controls row, with a horizontally-scrollable,
  uniformly-colored actions row and the same one-shot scroll-hint nudge
  the controls row uses.
- Redesign ShareUploadView to share the same SyncedHeaderScaffold chrome
  as every tab, with the uploading-file summary (marqueed via the new
  shared MarqueeTitle widget) and upload button grouped into one
  rounded/elevated bottom sheet.
- Restyle ShareSheet to open on the same DetailsHeader every other
  per-item sheet uses, and fix a couple of small design-language drifts
  (hardcoded text style, a non-rounded icon).
- Settings' account card: icon buttons with tooltips instead of text
  buttons for refresh/logout/remove, divider removed.

.claude/context/styling.md

@@ -42,6 +42,14 @@ widgets. Key points:
   `files_view.dart` and `share_upload_view.dart`'s destination picker).
   Extend this rather than re-deriving per-type icons/colors at a new call
   site.
+- [`DetailsHeader`](../../lib/widgets/details/details_sheet.dart) — the
+  icon-box/name/meta row every per-item bottom sheet opens on
+  (`DetailsSheet`, the media viewer's collapsed peek state, and
+  `ShareSheet`). Takes a `padding` override for callers whose own scroll
+  view already applies horizontal insets (`ShareSheet`'s `ListView`), so it
+  doesn't get doubled up. Reuse this instead of a bare title `Text` for any
+  new per-item sheet - a plain title reads as under-designed next to the
+  other sheets (a real instance: `ShareSheet` used to be just that).
 - [`FrostedGlassContainer`](../../lib/widgets/frosted_glass_container.dart) —
   the blurred/translucent pill background shared by all floating chrome
   (bottom nav bar, media-viewer top/bottom bars and video transport