feat(app): the UI foundation: theme, tabs and routes, strings

The app gets its look and navigation before more features land on them.

- Theme (core/theme/): hand-written light and dark Material 3 colour schemes around
  cobalt and copper; ThemeExtras for the colours Material has no role for (states, the
  four FSRS ratings); the 20 colours a person can give content. Tests hold text to WCAG
  AA contrast on each palette colour and on the dark theme's primary colour and surface.
- Layout and shared widgets: Material 3's window-size classes (core/layout/);
  SectionCard, AppLoadingIndicator, confirmDestructiveAction and FoundById
  (core/widget/). Lucide icons.
- Navigation with go_router: four tabs (Home, Library, Explore, Profile), each keeping
  its own stack of screens, as a navigation rail with the app's icon from 600 dp and as
  a navigation bar below; choosing the open tab again returns to its first screen. A
  studyset's screen sits in the library; studying, cards and settings cover the tabs.
- Routes carry ids: FoundById finds the item in the repository's live list, so a
  studyset deleted on another device, or an old link, says it no longer exists. A
  screen that deletes its own item keeps showing it while it closes, rather than
  flashing that message.
- Every person gets an app of their own (PersonApp, lib/src/app/): a
  MaterialApp.router with its own GoRouter and PersonScope around the navigator, built
  by SessionGate inside the person's keyed subtree. Screens read the person's
  repositories wherever they are built, so the static route() helpers and the listener
  that closed every pushed screen on a change of person are gone: a change of person
  now disposes the whole router and starts again at home.
- The account dialog becomes the Profile tab, with Settings (sign out, delete the
  account, About) behind a gear. A waiting GitHub sign-in now keeps waiting while
  another tab shows; a change of person still cancels it.
- The new screens' strings come from Flutter's gen-l10n (English for now). The
  generated files are committed, left out of analysis and formatting, and written
  anew by `flutter pub get`, so CI's check of generated code also catches stale ones.

Widget tests now start the whole app (test/support/pump_app.dart) at the screen's path
and move through it as a person would, by tabs and taps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

pubspec.lock

@@ -338,6 +338,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.0"
+  expressive_loading_indicator:
+    dependency: transitive
+    description:
+      name: expressive_loading_indicator
+      sha256: df84bb56f6f1e977df01e2455ccf033e89f92bc1ecf4dc78e71d44fcbc947331
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.0.1"
   fake_async:
     dependency: transitive
     description:
@@ -568,6 +576,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.2.0"
+  go_router:
+    dependency: transitive
+    description:
+      name: go_router
+      sha256: "530dcbb298dc7c319aea9e3b7c580a61effb2abee2c93825d2dd8d5af0e230f1"
+      url: "https://pub.dev"
+    source: hosted
+    version: "18.0.2"
   google_fonts:
     dependency: transitive
     description:
@@ -784,6 +800,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.3.0"
+  lucide_icons_flutter:
+    dependency: transitive
+    description:
+      name: lucide_icons_flutter
+      sha256: "50a21003cfe094e33b87e780d62ed5be84c32273dec1ba04547aab7df3cba00c"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.20"
   markdown:
     dependency: transitive
     description:
@@ -808,6 +832,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.13.0"
+  material_new_shapes:
+    dependency: transitive
+    description:
+      name: material_new_shapes
+      sha256: e4bc375205e187e8fb232573387112dd8c0dd45b03af8aa2b3c79eb4b9e3e0dc
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.0"
   material_ui:
     dependency: transitive
     description: