perf: fix severe slowdown with 5+ screenshots

[kmadhu0]

Closes #38

Problem

The app becomes almost unusable with more than 2-3 screenshots. Every slider drag triggered: a full synchronous canvas render (1320×2868 px), four additional side-preview renders at the same resolution, an immediate IndexedDB write, and a 14MB pixel-manipulation loop when noise was enabled. All of this on the main thread, per slider tick.

What changed

rAF batching + debounced save

• updateCanvas() now schedules via requestAnimationFrame, collapsing dozens of synchronous calls during a slider drag into a single render per frame
• saveState() is debounced to 500ms; exports and slide transitions use updateCanvasNow() to flush synchronously when needed

Side preview resolution

• Side-preview canvases now render at display resolution (~130px wide) instead of full export resolution (1320×2868), using ctx.scale() so font sizes and all other values stay correct — ~100× fewer pixels per preview

Side preview bitmap cache

• Non-selected screenshots don't change while you're editing the active one; their rendered bitmaps are cached and reused each frame instead of re-drawn. Cache invalidates on language/device switch, style transfer, and project change.

Canvas reallocation guard

• canvas.width = x unconditionally clears and reallocates the GPU buffer even when dimensions haven't changed; now skipped when unchanged

Noise

• Replaced getImageData → pixel loop → putImageData (14MB allocation per frame) with a cached offscreen canvas composited via overlay blend mode. Pattern only regenerates when intensity or canvas size changes.

Text normalization

• normalizeTextSettings() did a JSON deep-clone on every getText() call. A _normalized flag now short-circuits subsequent calls on the same object. Flag is stripped before IndexedDB serialization.

Blob URL leak

• getLucideImage() was creating a blob URL per icon render and never revoking it. Now revoked in both onload and onerror.

Testing

Verified with Playwright (headless Chromium):

• App loads without errors
• 3-4 screenshots added and navigated between
• Rapid slider input (20 events, no delay) → 0 errors, 1 render per rAF frame confirmed
• Side preview canvases confirmed at display resolution (not full export res)
• updateCanvasNow() + toDataURL() produces valid PNG for export
• Side preview cache confirms: 3 entries for 4 screenshots (1 active = uncached), stable across frames

Notes on PR #37

PR #37 includes the same revokeObjectURL blob URL fix — if that merges first, this line will be identical and conflict-free. All other changes here are unique to this PR. PR #37's "debounce" is a different mechanism (server-push to MCP backend, not IndexedDB).

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