Fix bugs across app + server, optimize UI/UX, add Gitea CI

Bug fixes (Flutter):
- Wrap multi-statement DB writes (insert/update/delete note, deleteDocument,
  deletePageData, OCR FTS merge, migrations) in transactions to prevent data
  loss on interruption and a read-modify-write FTS race.
- Fix PdfDocument leaks on exception (try/finally dispose) and preserve image
  aspect ratio when stamping images onto PDF pages.
- Guard file-picker against empty selection (was .single -> crash).
- Fix eraser ConcurrentModificationError and unmodifiable-list crash on PDF
  pages; capture page synchronously on save to stop wrong-page data loss.
- Fix Riverpod DB-not-ready races, broken pull-to-refresh, settings load race,
  and search N+1; transform stored annotations on PDF page rotation.
- Normalize pen pressure for devices without a pressure range.
- PPT: single source of truth for slide strokes so ink displays and exports.

UI/UX:
- Material 3 typography, theme-aware colors (dark-mode fixes), hover cursors
  and right-click/visible actions on desktop, keyboard shortcuts (undo/redo/
  save/find), toolbar overflow handling, friendlier empty states, semantic OCR
  status badges, relative timestamps, 1-based page indicators, large-deck PPT
  navigation, and a scratchpad-scope label in split view.

Server (optional backend):
- Persist JWT secret (was per-process random), block path traversal in storage,
  fix CORS '*'+credentials, add OCR job ownership checks, last-writer-wins sync
  guard, constant-time login, and split out heavy OCR deps so the API/tests run
  without them.

CI: Gitea workflows for format+analyze+test (Linux, system sqlite) and a
Windows release build; pristine `flutter analyze`, all Flutter and server tests
green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/utils/stroke_stabilizer.dart

@@ -0,0 +1,70 @@
+import '../models/ink_point.dart';
+
+/// Stabilization level for hand-drawn strokes.
+enum StabilizationLevel { none, light, medium, heavy }
+
+/// Smooths pen input using an Exponential Moving Average (EMA) filter.
+///
+/// - **none**: no smoothing (passthrough)
+/// - **light**: alpha = 0.6 (subtle smoothing)
+/// - **medium**: alpha = 0.4 (moderate smoothing)
+/// - **heavy**: alpha = 0.25 (strong smoothing, removes most tremor)
+///
+/// The formula applied to each coordinate independently:
+///   x_smoothed = alpha * x_raw + (1 - alpha) * x_prev
+class StrokeStabilizer {
+  final StabilizationLevel level;
+
+  double? _prevX;
+  double? _prevY;
+
+  StrokeStabilizer({this.level = StabilizationLevel.none});
+
+  double get _alpha {
+    switch (level) {
+      case StabilizationLevel.none:
+        return 1.0;
+      case StabilizationLevel.light:
+        return 0.6;
+      case StabilizationLevel.medium:
+        return 0.4;
+      case StabilizationLevel.heavy:
+        return 0.25;
+    }
+  }
+
+  /// Filters a raw point through the EMA, returning a smoothed point.
+  /// Returns the raw point unchanged if level is [StabilizationLevel.none].
+  InkPoint filter(InkPoint rawPoint) {
+    if (level == StabilizationLevel.none) return rawPoint;
+
+    final alpha = _alpha;
+
+    if (_prevX == null || _prevY == null) {
+      _prevX = rawPoint.x;
+      _prevY = rawPoint.y;
+      return rawPoint;
+    }
+
+    final smoothedX = alpha * rawPoint.x + (1 - alpha) * _prevX!;
+    final smoothedY = alpha * rawPoint.y + (1 - alpha) * _prevY!;
+
+    _prevX = smoothedX;
+    _prevY = smoothedY;
+
+    return InkPoint(
+      x: smoothedX,
+      y: smoothedY,
+      pressure: rawPoint.pressure,
+      tilt: rawPoint.tilt,
+      timestamp: rawPoint.timestamp,
+      pointerDeviceKind: rawPoint.pointerDeviceKind,
+    );
+  }
+
+  /// Resets the filter state. Call when starting a new stroke.
+  void reset() {
+    _prevX = null;
+    _prevY = null;
+  }
+}