fix(frontend): scrollable main region + collapse long run prompts

[iagop03]

Summary

Two related overflow bugs, both invisible with short content and only surfacing once content gets tall — a long generated report, or a long user prompt.

Why

1. Layout.tsx: <main> is a flex item (flex-1) inside a flex-col parent with overflow-hidden. Without min-h-0, a flex item's default min-height: auto lets it grow past its allotted space to fit tall content instead of respecting its own overflow-auto — the excess then gets hard-clipped by the parent's overflow-hidden, with no scrollbar at all, rather than scrolling into view. This affects every routed page, not just run detail — a long-running task's page (or any tall content) could become partially unreachable.
2. RunDetail.tsx: the header renders run.prompt in full, uncapped. That header sits above the scrollable body, so a long multi-paragraph prompt (a normal shape for this app — users write structured strategy specs) pushes the actual dashboard/tabs mostly off-screen, leaving only a thin scrollable strip below it.

Changes

• Layout.tsx: add min-h-0 to <main>.
• RunDetail.tsx: run.prompt now clamps to 3 lines by default (line-clamp-3) with a "show full prompt" / "show less" toggle button, collapsed by default.
• New i18n keys runDetail.showFullPrompt / runDetail.showLessPrompt across all 7 locales.
• Layout.test.tsx: regression assertion that <main> carries flex-1 min-h-0 overflow-auto.

Test Plan

• npx tsc -b — clean.
• Full frontend suite: npx vitest run --pool=threads — 594/596 passing; the 2 failures (options.test.ts ivLabel decimal-separator formatting, tearsheet.test.ts UTC-5 timezone-offset assumption) are pre-existing and locale/timezone-environment-dependent, unrelated to this change (reproduce identically on main).
• Targeted: Layout.test.tsx + RunDetail.test.tsx — 16/16 passing.
• Not tested against a live browser session manually; verified via the unit test's class assertions instead.

Checklist

• No changes to protected areas (src/agent/, src/session/, src/providers/)
• No hardcoded values
• Follows CONTRIBUTING.md (DCO sign-off on the commit)
• No user-facing docs needed — pure bugfix, no new config/flags/behavior to document