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-engine/src/pipeline.rs

@@ -4,12 +4,19 @@ use crate::lexer::tokenize;
 use crate::parser::parse;
 use crate::span::Span;
 use crate::types::CalcResult;
+use crate::variables::aggregators;
 
 /// Evaluate a single line of input and return the result.
 pub fn eval_line(input: &str, ctx: &mut EvalContext) -> CalcResult {
     let trimmed = input.trim();
     if trimmed.is_empty() {
-        return CalcResult::error("empty input", Span::new(0, 0));
+        return CalcResult::empty(Span::new(0, 0));
+    }
+
+    // Detect headings (# Title) and aggregator keywords (sum, total, etc.)
+    // before tokenizing — the lexer misinterprets `#` as a line reference prefix.
+    if aggregators::is_heading(trimmed) || aggregators::detect_aggregator(trimmed).is_some() {
+        return CalcResult::non_calculable(Span::new(0, trimmed.len()));
     }
 
     let tokens = tokenize(trimmed);
@@ -24,7 +31,7 @@ pub fn eval_line(input: &str, ctx: &mut EvalContext) -> CalcResult {
         )
     });
     if !has_expr {
-        return CalcResult::error("no expression found", Span::new(0, trimmed.len()));
+        return CalcResult::non_calculable(Span::new(0, trimmed.len()));
     }
 
     match parse(tokens) {