fix(ui): Tasks tab fills viewport height like the Chat tab (#1500)

[vybe]

Summary

• The Tasks tab joins FULLSCREEN_TABS (the Chat tab's h-screen flex-fill layout), so the task list absorbs the remaining viewport height instead of scrolling inside a fixed max-h-96 box on a mostly-empty page.
• Strict no-regression floor: the old max-h-96 list cap is retained as the history card's min-h-96 floor — no viewport height ever shows fewer task rows than before. Live measurement drove this: fixed chrome (~780px on a populated agent) would otherwise leave a 222px list at a 1000px viewport, fewer rows than the old cap.
• TasksPanel's root is both the fill column and the short-viewport fallback scroller (load-bearing: the page cannot scroll in fullscreen mode, so without it short viewports would hard-clip the composer). Padding moved onto the scroll root so the scrollbar sits at the card edge.
• Repairs the pre-existing Agent Detail: Chat tab content width jumps when switching to/from any other tab #954 spec (agent-detail-chat-width.spec.js): Agent Detail tabs: overflow into a "More" dropdown instead of horizontal scroll #1114's aria-hidden measuring mirror made the bare nav.-mb-px locator a strict-mode violation; the suite only runs on ui-labeled PRs, so the breakage was invisible until now (learnings entry added).

Changes

• src/frontend/src/views/AgentDetail.vue — FULLSCREEN_TABS = ['chat', 'tasks'] + panel contract comment; Tasks wrapper flex-fills
• src/frontend/src/components/TasksPanel.vue — fill-column restructure, min-h-96 floor, uncapped flex-1 list, vertically-centered loading/empty states, data-testid anchors
• src/frontend/e2e/agent-detail-tasks-fill-height.spec.js — new: floor ≥ old cap, growth-with-viewport (cap-removal signal robust to per-agent chrome height), page-scroll containment, Agent Detail: Chat tab content width jumps when switching to/from any other tab #954-class width parity, short-viewport composer reachability
• src/frontend/e2e/agent-detail-chat-width.spec.js — .first() repair
• docs/memory/feature-flows/{tasks-tab,session-tab}.md, feature-flows.md, learnings.md

Accepted tradeoffs (from plan review)

• Sparse data now shows a tall card with empty space below the rows (standard list-app pattern; empty state centers).
• Alternative "just remove the cap and let the page scroll" was rejected: it diverges from Chat and scrolls the composer/queue status away.
• Extreme viewport heights (<~350px) clip the AgentHeader — pre-existing fullscreen-mode behavior, exact Chat parity.

Test Plan

• e2e: npx playwright test e2e/agent-detail-tasks-fill-height.spec.js e2e/agent-detail-chat-width.spec.js — 5/5 passed against the live stack
• Live verification: card grows 382→620px across a 1000→1400px viewport; no page scrollbar; composer reachable via panel scroll at 600px; dark + light modes; empty state centered; populated list scrolls internally
• Existing behavior intact: ?execution= deep-link (scrollIntoView is container-agnostic), task expansion, ?tab=tasks deep-link

Fixes #1500

🤖 Generated with Claude Code

[dolho]

Review: ✅ Approve on approach — ⚠️ rebase required before merge

The layout approach is sound: FULLSCREEN_TABS = ['chat', 'tasks'], panel root as the flex-fill column and the short-viewport fallback scroller (load-bearing since the page can't scroll in fullscreen mode), and retaining the old max-h-96 cap as the new min-h-96 floor so no viewport ever shows fewer rows than before. The .first() repair of the #954 spec (broken by #1114's aria-hidden measuring mirror) is a legit pre-existing-rot fix, and the learnings.md entry on ui-gated e2e specs rotting silently is valuable.

Blockers before merge:

• Branch is CONFLICTING with dev — needs a rebase.
• No CI has run ("no checks reported on the branch"). After rebase, push so frontend-e2e actually executes — this is a ui-labeled PR and the new spec + Agent Detail: Chat tab content width jumps when switching to/from any other tab #954 repair only run there. Green CI on the fill/parity spec is the merge gate.

[github-actions]

⚠️ Nightly unit-suite check skipped — merge conflict against dev.

Resolve by running git merge dev locally and pushing the result. The next nightly run will re-test once the conflict is gone.

[dolho]

Review (direct). Tasks tab joins FULLSCREEN_TABS and flex-fills the viewport; old max-h-96 cap becomes the min-h-96 floor (no fewer rows at any height), panel root is the short-viewport fallback scroller; e2e fill/parity spec added + the #954 .first() locator repair. Rebased on dev, MERGEABLE; CI green except the ui-gated e2e still running — that's the gate for this one, wait for it. Sound approach. ✅ once e2e passes.