Add CLAUDE.md with topic-specific context docs

Split project context into .claude/context/ (architecture, server,
styling, standards) instead of one large file, referenced from a
minimal root CLAUDE.md.

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

.claude/context/architecture.md

@@ -0,0 +1,59 @@
+# Architecture
+
+## Layout
+
+```
+lib/
+  main.dart          # app root, theme wiring, top-level navigation switch
+  models/             # plain data classes (NextcloudItem, NextcloudShare, ...)
+  providers/           # ServerProvider — the single app-wide ChangeNotifier
+  services/             # network/IO: NextcloudService, LoginFlowService
+  theme/                 # AppTheme (Material 3 ThemeData)
+  views/                  # one screen each (FilesView, PhotosView, ...)
+  widgets/                 # reusable pieces shared across views
+    details/                # the file-details bottom sheet and its tabs
+```
+
+`views/` files are screens routed to directly (a tab, or pushed via
+`Navigator`). `widgets/` files are building blocks used by more than one
+view (or complex enough to warrant their own file) — nothing in `widgets/`
+owns app state itself; it reads it from the `ServerProvider` passed down or
+read via `context.watch`/`context.read`.
+
+## State management
+
+There is exactly one `ChangeNotifier`: [`ServerProvider`](../../lib/providers/server_provider.dart).
+It is created once in `main()` and provided at the root with `provider`'s
+`ChangeNotifierProvider`. It owns:
+
+- auth/session state (`isLoggedIn`, `isRestoringSession`, login-flow status)
+- the active `NextcloudService` instance (null until logged in)
+- all fetched data (`items`, `quota`, `activities`)
+- navigation-within-files state (`currentFolderPath`, `pathStack`)
+- UI settings that persist across launches (theme mode, seed color, dynamic
+  color toggle, bottom-bar opacity/blur, sort/filter/view-mode prefs)
+
+New app-wide state belongs on `ServerProvider` as a private field + getter +
+a method that mutates it and calls `notifyListeners()`. Screen-local state
+(e.g. a `TextEditingController`, an expanded/collapsed flag) stays in that
+view's own `State` class — see `standards.md` for the split.
+
+There's no separate repository/data layer: views call `ServerProvider`
+methods directly, which call `NextcloudService`/`LoginFlowService`.
+
+## Navigation / screen flow
+
+`main.dart`'s `NextcloudApp` picks the app's `home` screen from provider
+state, no named routes:
+
+- `provider.isRestoringSession` → `_SplashView` (spinner while
+  `flutter_secure_storage`/`shared_preferences` are read on startup)
+- else `!provider.isLoggedIn` → `LoginView` (server-address entry + Login
+  Flow v2)
+- else `MainShellView`
+
+`MainShellView` is a bottom-nav `IndexedStack` with three persistent tabs —
+Files, Photos, Activity — each keeping its own `ScrollController` so state
+(scroll position, `IndexedStack`'s built-but-hidden trees) survives tab
+switches. `SearchView` and the file-details sheet are pushed on top via
+`Navigator`/`showModalBottomSheet` rather than being tabs.