feat: add word-level intra-line diff highlighting

[jfyne]

What's changed

Adds word-level intra-line diff highlighting for paired del/add lines using a Myers diff algorithm on word-boundary tokens, with a 70% threshold guard, CSS highlight styles, and a fix for template whitespace rendering in code-text divs.

Risk Profile

Signal	Value	Risk
Size	711 LOC (+700, -11)	High
Files	7 files across 2 directories	Medium
Sensitive paths	None	Normal
Test coverage	3 test files / 4 production files	Moderate
Type	Feature	Normal

Overall: HIGH — size (711 LOC) sets floor to High; multiple medium signals (files, test coverage) would escalate but High is already the floor.

Suggested Review Order

#	File	What it does	Link
1	worddiff.go	Myers word-level diff algorithm: tokenizer, diff engine, and HTML renderer	View
2	view.go	Integrates intra-line diff into split and unified view builders	View
3	styles.css	CSS styles for .intra-del and .intra-add highlight spans	View
4	template.html	Fixes template whitespace in code-text divs using trim operators	View
5	worddiff_test.go	Unit tests for tokenizer, diff engine, rendering, and threshold logic	View
6	view_diff_split_test.go	Tests intra-line diff integration in split view (paired, unpaired, context)	View
7	view_diff_test.go	Tests intra-line diff integration in unified view (adjacent, non-adjacent)	View