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/architecture.md

@@ -113,15 +113,42 @@ owns the app's share-intent listener (`ShareIntentService`, backed by
 hand-rolled native handling in `MainActivity.kt` - see `server.md` for why
 this isn't the `receive_sharing_intent` plugin): both `getInitialShare()`
 (cold start via another app's "Share to...") and `onNewShare` (already
-running) push `ShareUploadView`.
+running) push `ShareUploadView`. It similarly owns the pick-intent listener
+(`PickIntentService` - see `server.md` for the full "being picked by
+another app" story) that feeds `ServerProvider.pickRequest`; while
+`isPicking`, the visible tab list is overridden to just Files and Photos
+regardless of the user's own hidden/reordered tab settings, since those
+are the only two views that know how to handle a picking-mode tap and the
+only two that make sense as external "choose a file" sources.
 
-Five of the six tabs (all but Files) plus each tab's own controls share
+All six tabs, plus `ShareUploadView` (the share-to-upload destination
+picker, pushed rather than a tab - see below), share
 [`SyncedHeaderScaffold`](../../lib/widgets/synced_header_scaffold.dart) — a
 `CustomScrollView` with a pull-down "sync status" header (Google
 Photos-style) and a classic Material refresh spinner shown during a
-pull-triggered sync. `SearchView`, `AccountView` (Settings), the
-file-details sheet, the share sheet, and `ShareUploadView` (the
-share-to-upload destination picker) are pushed on top via
+pull-triggered sync. `ShareUploadView` mirrors the Files tab's own
+controls-row/breadcrumbs sticky header almost exactly, so arriving via
+another app's "Share to..." sheet still lands on the same top chrome
+instead of a plain `AppBar` - its `actions` are the exact same
+`[MoreTabsButton(), ProfileAvatarButton()]` every non-Files tab uses (kept
+uniform deliberately; backing out is the system back gesture/button, not a
+bespoke close icon in the app bar). Its bottom action - "Upload to
+{folder}" - and the uploading-file-name summary above it (single line,
+auto-scrolling via `MarqueeTitle` if it doesn't fit) live together in one
+rounded-top, elevated `Material` bar as `bottomNavigationBar`, reading as a
+sheet peeking up from the bottom edge rather than a plain flat bar.
+[`MarqueeTitle`](../../lib/widgets/marquee_title.dart) (`package:marquee`) is
+shared with `FileViewerScreen`'s title - falls back to a plain ellipsized
+`Text` when the content already fits, so short text never marquees.
+`SearchView`, `AccountView` (Settings), the file-details sheet, and the
+share sheet are pushed on top via
 `Navigator`/`showModalBottomSheet`/`showGradualBottomSheet` rather than
 being tabs. `ProfileAvatarButton` (top-right on every tab) opens Settings on
 tap and cycles between saved accounts on a vertical swipe.
+
+Files and Photos (the two tabs with multi-select) pass their selection
+toolbar into `SyncedHeaderScaffold`'s `selectionBar` param rather than
+rendering it as a second sliver app bar inside their own content: while
+non-null, it fully takes over the pinned top bar in place of the
+sync-status chip/`actions`/pull-to-reveal quota panel, so selecting reads
+as replacing the whole top chrome rather than adding a strip beneath it.