feat(ui): scrollable help menu, Esc-to-close, + blush/matrix/wraith themes

The help overlay now scrolls (↑/↓, PgUp/PgDn, Home/End) with a position
indicator and only Esc dismisses it, so stray keystrokes can't close a menu
that overflows the screen. Adds three bundled vestments (blush, matrix,
wraith); they're auto-discovered via Theme::available(), so they appear in
the menu and /theme list with no hardcoded entries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hh/themes/blush.toml

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+# blush — soft femme: rose & lavender ink on a warm plum dusk
+name = "blush"
+bg = "#241622"          # warm deep-plum surface (not pure black)
+border = "#a76a8f"      # dusty mauve chrome
+title = "#ffd9ec"       # pale blossom
+accent = "#ff7fcf"      # bubblegum pink — ❀ glyph, prompt
+dim = "#9a7088"         # muted rose-grey timestamps
+me = "#ff9ed8"          # your messages — soft pink
+other = "#c89bff"       # others' messages — lavender
+system = "#ffb3d9"      # system lines — petal pink
+input = "#ff9ed8"
+roster_me = "#ff5fb0"   # you — vivid rose
+roster_width = 22
+sigil = "❀"             # blossom

hh/themes/matrix.toml

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+# matrix — digital rain: phosphor green cascading on the void
+name = "matrix"
+bg = "#000600"          # near-black with a green undertone
+border = "#0a5a1e"      # dim terminal-green chrome
+title = "#9dffb0"       # bright phosphor
+accent = "#00ff41"      # classic matrix green — glyph, prompt
+dim = "#1f7a33"         # faded rain trail
+me = "#00ff41"          # your messages — bright green
+other = "#39d353"       # others' messages — softer green
+system = "#7dffa0"      # system lines — pale phosphor
+input = "#00ff41"
+roster_me = "#aeff00"   # you — electric lime
+roster_width = 22
+sigil = "ⵣ"             # falling glyph

hh/themes/wraith.toml

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+# wraith — goth cyberpunk: blood & violet bleeding through obsidian
+name = "wraith"
+bg = "#0b0710"          # obsidian with a violet bruise
+border = "#5a2a4a"      # dark wine chrome
+title = "#e6d2ff"       # pale violet bone
+accent = "#b026ff"      # electric amethyst — ⸸ glyph, prompt
+dim = "#6a4a66"         # ashen mauve timestamps
+me = "#ff2b5e"          # your messages — arterial red
+other = "#9d4edd"       # others' messages — necro violet
+system = "#ff4f9a"      # system lines — toxic magenta
+input = "#ff2b5e"
+roster_me = "#ff1744"   # you — blood crimson
+roster_width = 22
+sigil = "⸸"             # inverted-cross dagger