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.md

@@ -22,3 +22,13 @@ relevant file(s) before working in that area rather than loading all of them:
 flutter test      # run tests
 flutter analyze   # static analysis / lints
 ```
+
+## Keeping docs current
+
+After making a change that affects architecture, server integration,
+styling conventions, or code standards, update the relevant file(s) in
+`.claude/context/` in the same session — don't leave documentation to drift
+from the code. This applies whether the change is a new feature, a
+refactor, or a convention shift (e.g. a new reusable widget, a changed
+state-management pattern, a new dependency). If a change doesn't fit any
+existing topic file, add a section rather than skipping the update.