feat(web): implement complete workspace with themes, tabs, sidebar, and mobile

Transform CalcText from a single-document calculator into a full workspace
application with multi-document support, theming, and responsive mobile experience.

- Theme system: 5 presets (Light, Dark, Matrix, Midnight, Warm) + accent colors
- Document model with localStorage persistence and auto-save
- Tab bar with keyboard shortcuts (Ctrl+N/W/Tab/1-9), rename, close
- Sidebar with search, recent, favorites, folders, templates, drag-and-drop
- 5 templates: Budget, Invoice, Unit Converter, Trip Planner, Loan Calculator
- Status bar with cursor position, engine status, dedication to Igor Cassel
- Results panel: type-specific colors, click-to-copy, error hints
- Format toolbar: H, B, I, //, color labels with live preview toggle
- Syntax highlighting using theme CSS variables
- Error hover tooltips
- Mobile: bottom results tray, sidebar drawer, touch targets, safe areas
- Docker multi-stage build (Rust WASM + Vite + Nginx)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

calcpad-macos/Tests/CalcPadTests/RustEngineTests.swift

@@ -105,12 +105,12 @@ struct RustEngineTests {
         #expect(result.isError == false)
     }
 
-    @Test("# is not a comment in Rust engine — treated as identifier lookup")
-    func hashNotComment() {
+    @Test("# heading is treated as non-calculable, not an error")
+    func hashHeading() {
         let result = engine.evaluateLine("# header")
-        // The Rust engine does not treat # as a comment. The # is skipped and
-        // "header" is parsed as an identifier, resulting in an undefined variable error.
-        #expect(result.isError == true)
+        // The Rust engine now detects headings (# followed by space) as non-calculable.
+        #expect(result.isError == false)
+        #expect(result.result == nil)
     }
 
     // MARK: - AC: Memory safety — no leaks with repeated calls