fix(feed): don't count inline media URLs toward the long-post threshold

[dmnyc]

Summary

A note with a short caption and an image gallery was collapsed behind a "Show more" toggle, with a large empty gap between the caption and the toggle — even though the actual text was only a few lines.

Root cause

PostCardView's long-post decision is isLong = pixelLong || charLong, where charLong = displayEvent.content.count > 600. That counts the raw content string, including inline media URLs. The repro note is 659 chars, but ~496 of those are five inline .png image URLs (~90 chars each) that render as the gallery, not as text — the caption is only ~163 chars / 4 lines. charLong tripped, the post was collapsed with the 280pt text cap, and since the text is far shorter than 280pt the frame left a big empty gap above "Show more".

Fix

• Add ContentParser.textualLength(content:tags:) — sums only the text-bearing segments (.text, .hashtag, .inlineLink), excluding media, standalone link-preview cards, embedded quote cards, and lightning-invoice cards.
• charLong now requires both the raw count and the text-only length to exceed 600. The raw-count guard keeps the extra parse off the hot path for short posts.
• Genuinely long text posts (600+ chars of actual text) still collapse; the measured-height (pixelLong) path is unchanged.

Fixes #405.

Before / After

Before	After
Needless "Show more" + empty gap on a short gallery caption	Full caption, no truncation, gallery directly below

Test plan

• Build succeeds, installed and ran on simulator
• Repro note renders its full caption with no "Show more" and no empty gap (verified)
• A genuinely long (600+ chars of text) post still collapses with "Show more"
• A gallery post with a long caption AND many images truncates on the caption length, not the URL count