Raise inline transcription media limit to 50MB and centralize limit constants

[Copilot]

The transcription flow was enforcing a hardcoded 20MB inline media cap and surfacing that value in user-facing errors. This updates the local guardrail to 50MB and makes the limit configurable via named constants.

• Inline media limit refactor

  • Introduced MAX_INLINE_MEDIA_MB and derived MAX_INLINE_MEDIA_BYTES in services/aiService.ts.
  • Replaced hardcoded size checks with the shared byte constant.

• User-facing messaging alignment

  • Updated the size-limit error text to reference the constant-backed MB value, removing stale 20MB text.

• Codepath cleanup in touched area

  • Removed duplicated inline size-check logic in the same file and kept enforcement in transcribeAudio().

const MAX_INLINE_MEDIA_MB = 50;
const MAX_INLINE_MEDIA_BYTES = MAX_INLINE_MEDIA_MB * 1024 * 1024;

if (file.size > MAX_INLINE_MEDIA_BYTES) {
  throw new Error(
    `File size exceeds ${MAX_INLINE_MEDIA_MB}MB limit for inline media. Please use a smaller file or split the audio.`
  );
}