perf(editor): dedupe alignment snap-lines in O(n) instead of O(n²)

[ly-wang19]

What & why

uniqAlignLines (lib/utils/element.ts) merges alignment snap-lines with Array.findIndex inside a forEach — O(n²):

lines.forEach((line) => {
  const index = uniqLines.findIndex((_line) => _line.value === line.value); // O(n)
  ...
});

It runs on every drag/scale mousemove (useDragElement / useScaleElement), and the snap-line count grows with the number of nearby element edges. On element-dense canvases that’s ~n² comparisons per pointer move at ~60 Hz → drag jank.

Closes #691.

Fix

Dedupe in O(n) via a Map keyed on value. A Map preserves first-occurrence insertion order (re-set-ting an existing key keeps its slot), so the output order and the range-merge semantics are unchanged — behavior-preserving, just linear.

Test plan

New tests/utils/uniq-align-lines.test.ts (5 cases): dedupe + outer-bound range merge, first-occurrence order preserved, single line, empty input, and many-duplicates collapse. tsc/prettier/eslint clean.

No user-facing strings (no i18n impact). (Separate test file from element.test.ts to avoid overlap with #675, which also touches element.ts.)