fix(composer): scale strokes, anchor lines, snapshot state changes

Three review findings in the composer's JavaScript, all confirmed against the
code.

Stroke widths did not scale with SCALE
--------------------------------------
_drawElement scales all geometry by `s`, but left ctx.lineWidth in canvas
pixels, so at SCALE>1 every outline rendered thinner than one LED pixel and
the preview stopped matching the panel it is previewing. Fixed for rectangle,
ellipse, arc, rounded_rectangle, line, divider and progress_bar. Ellipse and
arc also inset their radii by half the scaled width -- a stroke straddles its
path, so without the inset the outline spills outside the element's bounds.
The gauge branch already did this; the rest now match it.

Selection handles and the grid stay in canvas pixels deliberately: they are
editor chrome, not LED geometry, and live in other functions.

`line` ignored anchors
----------------------
_drawElement resolves ax/ay for every element, but the line branch drew raw
el.x0/el.y0/el.x1/el.y1. Setting xAnchor or yAnchor moved every other element
type and left lines where they were. getBoundingBox had the same omission, so
even once a line moved its hit box would not have. Both now translate by
(ax - el.x0, ay - el.y0); ax resolves from el.x0 for a line, so that is
exactly the anchor offset.

Four state mutations skipped _snapshot
--------------------------------------
_snapshot serialises metadata and currentPreset and is the only caller of
_debouncedAutosave. onBgColorChange, setCustomSize, changePreset and
applyPresetLabel each changed exactly those values without calling it, so the
background colour and the canvas size were lost on reload and could not be
undone. Same defect already fixed in onColorChange.

The review named three; applyPresetLabel has it too -- it is the branch that
handles sizes absent from DISPLAY_PRESETS.

Snapshotting is on the user-driven path only. _applyState and loadTemplate
drive these with {silent: true} while restoring, and snapshotting there would
push restore steps onto the undo stack and re-autosave the state just loaded.

Tests
-----
No JS runner here, so test_composer_js_contracts.py asserts on the parse tree
via tree-sitter: both files parse, no bare `ctx.lineWidth = 1` inside
_drawElement, the line branch and its bounding box carry the anchor offset,
each of the five mutations snapshots, and the two preset paths keep their
!opts.silent guard ahead of the snapshot.

9 of its 11 checks fail against the previous JS. Full suite 3978 passed, the
one failure being test_install_lowmem (pre-existing, awaiting #492).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

web_interface/static/v3/js/composer/composer-app.js

@@ -480,7 +480,11 @@ function composerApp() {
       this.currentPreset = `${w}×${h}`;
       this.SCALE = w <= 64 ? 6 : w <= 128 ? 4 : 2;
       this._applyScale();
-      if (!opts.silent) this.render();
+      if (!opts.silent) {
+        this.isDirty = true;
+        this._snapshot();
+        this.render();
+      }
     },
 
     changePreset(presetLabel, opts = {}) {
@@ -496,7 +500,11 @@ function composerApp() {
         canvas.style.width  = (this.MATRIX_W * this.SCALE) + 'px';
         canvas.style.height = (this.MATRIX_H * this.SCALE) + 'px';
       }
-      if (!opts.silent) this.render();
+      if (!opts.silent) {
+        this.isDirty = true;
+        this._snapshot();
+        this.render();
+      }
     },
 
     // ── Rendering ─────────────────────────────────────────────────────
@@ -626,6 +634,10 @@ function composerApp() {
       this.currentPreset = `${w}×${h}`;
       this.SCALE = w <= 64 ? 6 : w <= 128 ? 4 : 2;
       this._applyScale();
+      // currentPreset is part of the snapshot; a custom size set here was
+      // otherwise dropped on reload and could not be undone.
+      this.isDirty = true;
+      this._snapshot();
       this._setStatus(`Canvas set to ${w}×${h}`, 'info');
     },
     zoomFit() {
@@ -645,6 +657,11 @@ function composerApp() {
         g: parseInt(hex.slice(3, 5), 16),
         b: parseInt(hex.slice(5, 7), 16),
       };
+      // _snapshot serialises metadata and is the only caller of
+      // _debouncedAutosave, so without this the background colour was lost on
+      // reload and could not be undone. Same defect as onColorChange.
+      this.isDirty = true;
+      this._snapshot();
       this.render();
     },