feat(gui): add Sessions & Connections section

[kianwoon]

Summary

• Add a new "Sessions" section to the Tauri GUI showing active sessions, connection counts per provider, request stats, and last-seen time
• Uses existing sessionStats from WebSocket summary messages — no new backend endpoints needed
• Keyed DOM diffing (sessionRows Map) for efficient real-time updates

Changes

• index.html: Add #sessions-section between Providers and Recent Requests
• app.js: Add renderSessions() with keyed DOM diffing, DOM refs, section ordering
• styles.css: Add session card styles, fix flex layout (flex-shrink: 0 removed), fix compact mode transitions (max-height: none → 2000px), show 2 sections in compact mode
• tauri.conf.json: Increase window height 800→960, compact height 420→560

Test plan

• Launch GUI — Sessions section appears between Providers and Recent Requests
• Active sessions show truncated ID, connection count badge, request count, last-seen time
• Session rows update in real-time as requests flow through
• Compact mode shows 2 sections (Models + Providers) with proper height
• Toggle compact mode on/off preserves all section visibility

[kianwoon]

CI Failure — Not related to this PR

The failing test emits SSE error event when body stalls after headers received (tests/proxy.test.ts:398) is a pre-existing flaky timeout test in the stall detection suite:

• Node 20: PASS ✅
• Node 22: FAIL ❌ (20s timeout)

This test was not modified by this PR. The PR only changes GUI frontend files (HTML/CSS/JS + Tauri config) — no backend or test changes.

Rerunning the CI job should pass.