fix(post): remove max_chars budget cap for ASCII art

[thedarkknight197]

Problem

When publishing a post with an image, _convert_image_to_art capped the art to the remaining character budget (max_chars - text_len - 20). For a typical post with ~100 chars of text, this left ~860 chars for the art.

With a portrait photo (e.g. 3024×4032), the max_chars constraint in image_to_ascii shrinks width iteratively until the art fits — converging to ~38 chars wide. The result is unrecognizable noise.

Fix

Remove the budget constraint entirely. ASCII art is already exempt from the post character limit (added in #13: has_image check skips the max_chars validation on text). There's no reason to cap the art separately — it lives in an HTML comment and doesn't affect readability.

Art now generates at full quality: 80 chars wide by default.

Before vs After

Before (38 chars wide — unrecognizable):

::,~-{{1t|)t\>:IIllll!!!!iiiiiiii_|{
,,^}[_)[r{\(\1;;IIIllllll!!!!!i!>[1{
""^{?_1{x}\)/fI:;;IIIIIlllllll!I~{}1
^^^1-_{1r[\1//;,::;;;IIIIIIIIII;i[}1

After (80 chars wide — Ferrari + person + Dubai skyline clearly visible):

::::::::1;if|)[]\\ft|j1{fr/)t?,IIIIIlllllll!!!!!!!!iiiiiiiiiiiiiiii>>>>>>}]?jt}(
:,:::::~[-{]}1(]{\//)\([|rt1/-,I;IIIIIllllllllllll!!!!!!!!!iiiiiiiiiiiii><l|1)}}
,,,,:,\{))?,[|/+_jrr1?|/\||1/1;:;;IIIIIIlllllllllll!!!!!!!!!!!!!!!iiiiiii,](})1[
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,",,,^`)))_;}|\~-jrr1](/\)1(xj\<,;;;;IIIIIIIIIIllllllllllllllll!!!!!!!!I]1[}}{([
"""",^^(1(+;}||~}rrr1?(/\11(rj|>,::;;;;;IIIIIIIIIllllllllllllllllllllllI<}[[{}([
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