fix(text-editor): fix heading hierarchy and prose spacing

[ComputerCrack]

Problem

The editor's typography had several issues:

h1–>h3 only!
h4–>h6 had no styles, falling back to browser defaults which were visually
indistinguishable from body text
150% line-height on headings
semantically correct but visually too loose for large font sizes;
tight letter-spacing on display sizes is standard practice
Double paragraph margins
margin-block-start + margin-block-end on < p > combined with margin-top on >+ produced inconsistent spacing between paragraphs and other block elements
Horizontal rule
1px, no vertical breathing room, visually weak
List bullets misaligned
::before pseudo-element had a fixed line-height: 1.65 while positioned
absolute, causing bullets to sit higher than the text baseline
List indent too narrow
0.75rem made nested content feel cramped

Changes

h1–>h6
fully defined with proportional font-size, line-height, letter-spacing and margins
following a clear visual scale
Paragraph spacing moved entirely to >+ (owl selector)
single source of truth for block spacing, no more margin collapsing issues
HR: 2px, 1.5rem vertical margin
List bullet ::before: line-height: inherit + top: 0
bullets now vertically centered with text at all font sizes
List indent increased to 1.5rem
prose.scss aligned to the same system

[huly-github-staging]

Connected to Huly®: UBERF-15852