Release v2.1.1 — exchange-scoped chart overlays, mobile layout, demo video

[Stenvro]

Fixes

• Chart overlays are scoped to the chart's exchange — a bot's signals, trades and engine thoughts no longer show up on the same pair on a different exchange. /api/bots/signals takes an exchange parameter; bot list/summary expose the effective candle exchange (API key's exchange when routed, else data_exchange). The overlay menu now explains when a bot on this pair runs on another exchange.
• Stale chart tabs are closed automatically — tabs whose candles were deleted are dropped on login and after any delete in the Data Vault (view falls back to Home).
• Data Vault deletes are scoped to the exchange — deleting BTC/USDC 1d no longer wipes that pair on every exchange.

UI

• Mobile: the menu button now lives inside the page header (no more floating button over the title); Home gets the same slim bar on phones; section headers wrap their actions under the title instead of truncating.
• Analytics: filter panel collapses to a one-line summary on phones and is no longer sticky there; desktop unchanged.

Docs

• Demo video (24 s, with sound) at the top of the README, plus screenshots.

Tests

• test_chart_signals_and_bot_exchange_are_scoped_per_exchange added; 82 tests green, ruff + frontend lint/build clean.