feat(canvas): compute real boolean shape geometry with paper.js

Add a booleanShapePath engine that runs union/subtract/intersect/
exclude/flatten through paper.js and returns normalized result path
data plus per-operand stroke segments, so boolean results are real
vector paths instead of SVG mask approximations.

- Store the computed pathData on boolean groups and recompute it when
  the operation changes; flatten legacy groups into ordinary path nodes
- Render previews and SVG/PSD exports from the computed result path,
  keeping each operand's stroke style on the boundary it contributes
- Open boolean results in the path editor and flatten them on commit
- Add paper 0.12.18 with a paper-core type shim and mark it as a
  server-external package for the standalone Next build

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

frontend/pnpm-lock.yaml

@@ -58,6 +58,9 @@ dependencies:
   next:
     specifier: ^16.2.10
     version: 16.2.10(react-dom@19.2.7)(react@19.2.7)
+  paper:
+    specifier: 0.12.18
+    version: 0.12.18
   react:
     specifier: ^19.2.7
     version: 19.2.7
@@ -1988,6 +1991,11 @@ packages:
     resolution: {integrity: sha512-w+eufiZ1WuJYgPXbV/PO3NCMEc3xqylkKHzp8bxp1uW4qaSNQUkwmLLEc3kKsfz8lpV1F8Ht3U1Cm+9Srog2ug==}
     dev: false
 
+  /paper@0.12.18:
+    resolution: {integrity: sha512-ZSLIEejQTJZuYHhSSqAf4jXOnii0kPhCJGAnYAANtdS72aNwXJ9cP95tZHgq1tnNpvEwgQhggy+4OarviqTCGw==}
+    engines: {node: '>=8.0.0'}
+    dev: false
+
   /picocolors@1.1.1:
     resolution: {integrity: sha512-xceH2snhtb5M9liqDsmEw56le376mTZkEX/jEb/RxNFyegNul7eNslCXP9FDj/Lcu0X8KEyMceP2ntpaHrDEVA==}