fix(tui): third audit — search excerpts, narrow widths, NO_COLOR, grid cursor

[m1ngsama]

A third audit found search excerpts that never showed the query, a few renderers overrunning the frame at ≤60 columns, and NO_COLOR ignored.

• Docs search excerpts are windowed around the first occurrence of the query to fit the column, with "…" on either side, so every result shows why it matched.
• The heatmap, term-density chart, next-class/next-event banners and weekday strip use the same body width as everything else; a test asserts nothing passes the rule at 40 and 60 columns in zh and en.
• NO_COLOR (any non-empty value) turns colour off in auto mode; an explicit "on" still forces it.
• ⏎ on an empty cell in the full week grid stays in the grid; the grid cursor no longer truncates the course name.
• Separators follow the icon mode ("-" in ASCII); events search results read "“meeting”的搜索结果 · 9 场"; zh running text uses full-width colons; placeholders end in "..."; input footers read "⏎ confirm · Esc back · Ctrl+C quit"; archived-year counts are right-aligned; the browser fallback is one sentence per language.