perf(web): skip base64 for oversized image candidates - #5220

Note

Low Risk
Localized performance change in client-side image encoding; no auth or persistence contract changes beyond failing when no candidate fits instead of returning an over-budget data URL from one encode pass.

Overview
Image compression in imageCompression.ts avoids expensive base64 work when an encoded candidate is already over the character budget.

encodeToDataUrl becomes encodeCanvas with a budgetChars argument. On the HTML canvas path it only keeps toDataURL output when dataUrl.length <= budgetChars. On OffscreenCanvas, it estimates data URL length from the blob size and skips blobToDataUrl when that estimate exceeds the budget, returning { dataUrl: null, mimeType } instead.

encodeWithinBudget no longer tracks the smallest over-budget encoding across quality steps. It returns the first encoding that fits, or null if every attempt in that pass is too large—so callers rely on smaller fallback scale passes in reencodeWithinBudget rather than accepting a single pass that still overflows the budget.

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Note

Skip base64 encoding for oversized image candidates in encodeWithinBudget

• encodeCanvas in imageCompression.ts now accepts a budgetChars parameter and returns null for dataUrl when the encoded size exceeds the budget, avoiding unnecessary base64 conversion for large blobs.
• encodeWithinBudget now returns null when no encoding fits within the budget, rather than falling back to the smallest over-budget encoding.
• Behavioral Change: callers that previously received an over-budget data URL will now receive null when no candidate fits.

^{Macroscope summarized f91ad4f.}