fix: spatial index, decimation overflow, input validation, accessibility

Round 2 of performance and correctness improvements:

- Spatial grid index for brush painting: forEachTriInSphere now queries
  only nearby grid cells instead of scanning all triangles. ~5.7x faster
  for brush operations on 68k+ tri meshes.

- Decimation overflow fix: hasLinkViolation used a fixed 0x200000
  multiplier for vertex-pair keys, overflowing at >2M vertices.
  Now uses dynamic multiplier based on actual vertex count.

- Decimation determinant threshold: solveQ used absolute 1e-10 which
  fails for large coordinates. Now relative to matrix element magnitude.

- 3MF triangle index validation: bounds-check all parsed indices against
  vertex count, throw clear error on corrupt files instead of silent NaN.

- File size limit: reject files >500 MB before loading into memory,
  prevents browser tab crash on oversized files.

- Accessibility: preset swatches now keyboard-navigable (role=button,
  tabindex=0, Enter/Space to select). Modal dialogs trap focus and
  close on Escape.

- Ctrl+click straight line tool: click to set start point, Ctrl+click
  to paint a straight line between points. Ctrl+hover shows preview.

- Precision masking available for radius brush mode.

- Spatial grid rebuilt when entering/leaving precision mode.

js/stlLoader.js

@@ -3,6 +3,8 @@ import { OBJLoader } from 'three/addons/loaders/OBJLoader.js';
 import { unzipSync } from 'fflate';
 import * as THREE from 'three';
 
+const MAX_FILE_SIZE = 500 * 1024 * 1024; // 500 MB
+
 const stlLoader = new STLLoader();
 const objLoader = new OBJLoader();
 
@@ -12,6 +14,11 @@ const objLoader = new OBJLoader();
  * The geometry is translated so its bounding-box centre is at the world origin.
  */
 export function loadSTLFile(file) {
+  if (file.size > MAX_FILE_SIZE) {
+    return Promise.reject(new Error(
+      'File too large (' + Math.round(file.size / 1024 / 1024) + ' MB). Maximum supported: ' + (MAX_FILE_SIZE / 1024 / 1024) + ' MB.'
+    ));
+  }
   return new Promise((resolve, reject) => {
     const reader = new FileReader();
     reader.onload = (e) => {
@@ -73,6 +80,11 @@ export function getTriangleCount(geometry) {
  * Returns { geometry, bounds }.
  */
 export function loadOBJFile(file) {
+  if (file.size > MAX_FILE_SIZE) {
+    return Promise.reject(new Error(
+      'File too large (' + Math.round(file.size / 1024 / 1024) + ' MB). Maximum supported: ' + (MAX_FILE_SIZE / 1024 / 1024) + ' MB.'
+    ));
+  }
   return new Promise((resolve, reject) => {
     const reader = new FileReader();
     reader.onload = (e) => {
@@ -99,6 +111,11 @@ export function loadOBJFile(file) {
  * Returns { geometry, bounds }.
  */
 export function load3MFFile(file) {
+  if (file.size > MAX_FILE_SIZE) {
+    return Promise.reject(new Error(
+      'File too large (' + Math.round(file.size / 1024 / 1024) + ' MB). Maximum supported: ' + (MAX_FILE_SIZE / 1024 / 1024) + ' MB.'
+    ));
+  }
   return new Promise((resolve, reject) => {
     const reader = new FileReader();
     reader.onload = (e) => {
@@ -170,6 +187,13 @@ function parse3MF(data) {
         triangles[i * 3 + 2] = parseInt(triEls[i].getAttribute('v3'), 10);
       }
 
+      const vertCount = vertEls.length;
+      for (let i = 0; i < triangles.length; i++) {
+        if (triangles[i] < 0 || triangles[i] >= vertCount || isNaN(triangles[i])) {
+          throw new Error('Invalid triangle index in 3MF file');
+        }
+      }
+
       // Normalise path for lookup (strip leading slash, use forward slashes)
       const normPath = path.replace(/^\//, '').replace(/\\/g, '/');
       objectMap.set(normPath + '#' + id, { vertices, triangles });