fix(portal): language list fits in English; themed scrollbars; new README screenshots (#26)

[rbferreira]

What was wrong / what was missing

• In English the sidebar language list was cut to "Engl…" — LANGUAGE is wider than IDIOMA, leaving the compact list too little room.
• Scrollbars were the browser's default wide grey bar on every page and inner scroll area.
• The README screenshots predated the theme switch and the form-control standard.

What this changes

• sg-field-sm.sg-select keeps less padding around the chevron, and the sidebar list takes the rest of its row (flex-1).
• Themed scrollbars in index.css: scrollbar-width: thin + scrollbar-color (Firefox, current Chromium) and ::-webkit-scrollbar rules (Safari, older Chromium) — transparent track, rounded thumb in the border shade, both themes.
• The five README screenshots retaken from a real cluster: addresses replaced by documentation ranges (192.168.0.x, 2001:db8::), torrent names by public ISOs and open films, no tracker names.

How it was verified

• Clean node:22 container: tsc --noEmit, vitest (733 passing), npm run build.
• Checked in a browser at 1600px: the list reads "English" in full; the page scrollbar is thin and dark.

Checklist

• Documentation updated in this pull request (or nothing documented changed) — README screenshots
• CHANGELOG.md updated under [Unreleased], if users will notice the change
• ADR written first, if the change is architectural — not architectural
• Database changes only through a new migration — none
• No secrets, private addresses or machine names in code, tests or docs

Closes #26

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https://claude.ai/code/session_01Tk9HuwXZP8CdJu5vRLaoBq