Add multi-account support and update project docs

- Add SavedAccount/AccountStore and rework ServerProvider around a list
  of saved accounts: switch/remove/add, swipe-to-cycle on the avatar
  button, per-account browsing prefs (sort/view/filters/cache), and a
  session-generation guard so a stale fetch from an abandoned account
  can't land in the newly active one
- Migrate existing single-account installs into the new schema
  automatically so upgrading users stay logged in
- Settings gains an Accounts section; LoginView gains an "add account"
  mode pushed on top of the main shell instead of replacing it
- Give every Scrollbar in the app a thicker, draggable, Android-style
  thumb via a shared ScrollbarTheme instead of the default hairline
- Splash screen: use the monochrome app icon (tinted per theme) and a
  smaller spinner
- Update CLAUDE.md and .claude/context/*.md to reflect the accumulated
  multi-account, sharing, and UI changes across this session, and add
  a standing instruction to keep these docs current going forward

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.claude/context/standards.md

@@ -33,6 +33,25 @@ class/method already makes obvious.
   and `context.read<ServerProvider>()` for one-off calls from callbacks
   (matches `LoginView._handleContinue`).
 
+## `ServerProvider` conventions
+
+- Any method that fetches data and writes it into a shared field
+  (`refreshData`, `fetchAllMedia`, `fetchTrash`, `fetchShares`,
+  `fetchRecent`, `_applyCredentialsForAccount`) must guard against a stale
+  write from an account the user has since switched away from: capture
+  `final gen = _sessionGeneration;` at entry, and check
+  `if (gen != _sessionGeneration) return;` immediately after each `await`
+  before touching any field or calling `notifyListeners()`. Follow this
+  pattern for any new fetch method added to the provider.
+- New persisted state on `ServerProvider` must be classified global vs.
+  per-account (see `architecture.md`/`server.md`) up front — global state
+  uses a plain `_prefsFuture.then((p) => p.setX(key, value))`; per-account
+  state goes through `_persistAccountPref(key, (p, namespacedKey) =>
+  p.setX(namespacedKey, value))` and must also be handled in
+  `_applyAccountPrefs` (both the "reset to default when no account" and the
+  "load for this account" branches) so it's correct immediately after a
+  switch, not just at startup.
+
 ## Testing
 
 - Widget tests must mock platform channels that the app touches on startup