[FEATURE] Paper mode in hub: blue palette, banner, sample runner - closes #86

[Ibrahim-3d]

Summary

Wires paper trading into the hub UI per @sjackson0109's feedback on #90 and the pivot proposed there.

• File menu gains a Paper Mode checkbutton; selection persists in gui_settings.json
• Paper mode paints the hub with a blue palette instead of black
• PAPER TRADING banner across the top + contrasting label in the Current Trades panel
• Banner hosts a Run sample trade button that pulls a live BTC price from Binance's public ticker and routes a simulated buy then sell through PaperTradingAccount
• Drops the duplicate standalone demo_paper_trading() stub from pt_paper_trading.py

Supersedes #90 (the standalone demo script approach) and closes #86.

Why this pivot

From #90 conversation:

sjackson0109: "How about we consider paper mode changes the CSS of the background. Say BLUE instead of BLACK. So it is blatantly obvious which mode we are in. And in the trading section, add the words 'PAPER TRADING'. In a contrasting font colour. The file menu should have the 'paper mode' toggle button."

The standalone script in #90 worked but lived outside the hub, so newcomers had to know about the CLI. This PR moves the same scenario behind a button inside the hub and adds the visual safety rails (blue palette + banner + label) so it is impossible to confuse paper mode for live trading.

Implementation notes

• New module app/pt_paper_mode.py owns the palette, banner widget, sample runner, and disk-read helper. Keeps pt_hub.py edits surgical (~190 lines added, all isolated to the menu/theme entry points).
• Paper-mode flag is read straight off disk before the first _apply_forced_dark_mode call so the window does not flash dark before settling on blue.
• The toggle shows a Restart for full effect notice because matplotlib chart facecolors and option_add classic-widget defaults are baked at widget-creation time. Banner and trades-section label toggle live without restart.
• Binance fetch is a 5s public GET with a deterministic fallback path; tests cover happy/missing-key/network-error paths and the buy/sell scenario with and without a live price.

Files

File	Change
app/pt_paper_mode.py	new - palette, banner, sample runner, persistence helper
app/pt_hub.py	settings key, settings-load reorder, palette overlay branch, file-menu toggle, banner build, trades-section label, sample-runner handler
app/pt_paper_trading.py	removed duplicate async demo_paper_trading() stub
app/test_paper_mode.py	new - 14 tests covering palette mirror, disk round-trip, fetch happy/error paths, sample runner with live price + fallback

Local checks

• python -m black --check app/ clean (102 files unchanged)
• python -m flake8 app/pt_paper_mode.py app/test_paper_mode.py --max-line-length=100 --extend-ignore=E203,W503 clean
• python -m flake8 app/pt_hub.py --select=E9,F63,F7,F82 clean (hard-error gate)
• python -m pytest app/test_paper_mode.py app/test_paper_trading_integration.py -q -> 22 passed

Test plan

• Toggle File > Paper Mode off then on - check gui_settings.json flips and a restart paints blue with banner + Current Trades label
• Click Run sample trade with internet on - banner shows live BTC, buy/sell delta is small (commission + drift)
• Click Run sample trade with internet off - banner shows offline (using sim), trade still fills via simulator
• Toggle paper mode off mid-session - banner and label disappear immediately, restart prompt shown

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[Ibrahim-3d]

Closing - work belongs on the existing PR #90 branch per author. Cherry-picking the commit there.