Session History Log + Dark Mode Table Fix

[SeanathanVT]

Summary

Implements persistent session history with a responsive recent-sessions table, CSV export, and history clearing on the start screen. Also fixes the session history table to properly support the Light/Dark theme toggle.

---

What's New

Session History Log

• Persistent storage — Completed sessions are saved to session_history.json as structured JSON records containing: date, start/end time, duration, distance (km/mi), steps, calories, and average speed (km/h and mph).
• End Session button — Red "End Session" button on Active and Paused screens explicitly ends a session, stops the belt, saves stats to history, resets all counters, and returns to the start screen.
• Graceful shutdown hook — In-progress sessions are automatically captured on graceful shutdown (Ctrl+C or Close button) so no data is lost.
• Thread-safe file I/O — All reads/writes protected by threading.Lock() (_history_lock) to prevent corruption across Flask request threads and the BLE thread.
• Fault tolerance — Corrupted or missing session_history.json is handled gracefully — starts fresh with an empty array and logs a warning.

Recent Sessions Table

• Start screen displays the last 10 sessions (configurable via HISTORY_DISPLAY_LIMIT) in a responsive table showing date, time, duration, distance, steps, calories, and average speed.
• Fully styled with custom CSS using CSS custom properties — no Bootstrap .table class dependency.

CSV Export & Clear History

• Export CSV — /export_csv route generates a downloadable walkingdad_history.csv containing all historical sessions (full history, no limit).
• Clear History — Button with confirmation dialog that truncates the history file via /clear_history POST endpoint.

---

Bugfixes

Session History Table Dark Mode

• Table background was not following the Light/Dark theme toggle, causing white-on-white (invisible) text in dark mode.
• Rewrote all table styling from scratch using CSS custom properties (--card-color, --text-color, --border-color, --button-hover-bg) so every color — background, borders, hover, text — properly switches with the theme toggle.

---

Files Changed

File	Changes
app.py	Added _build_session_record(), _save_session(), _load_session_history(), _clear_session_history(), _load_full_session_history() functions. Added /end_session, /export_csv, /clear_history routes. History loaded in root(). Shutdown hook calls _save_session(). New constants: HISTORY_FILE, HISTORY_DISPLAY_LIMIT, _history_lock.
templates/start_session.html	Added End Session button, recent sessions table with .history-table class, Export CSV and Clear buttons, clear history JavaScript handler.
templates/active_session.html	Added red "End Session" button below speed controls.
templates/paused_session.html	Added red "End Session" button next to Resume.
templates/base.html	Added .history-table custom CSS rules with dark mode support.
.gitignore	session_history.json exclusion (already present from prior work).
CHANGELOG.md	Updated to v1.2.0, restructured release entries.
ROADMAP.md	Marked item 2.5 as Complete (v1.2.0).

---

Testing Checklist

• Start and complete a session → verify record appears in Recent Sessions table
• End session via button → verify belt stops, session saved, counters reset, redirected to start screen
• Ctrl+C during active session → verify session_history.json contains the session after exit
• Export CSV → verify all sessions included (not just last 10)
• Clear History → confirm dialog appears, table empty afterward, history file truncated
• Toggle Dark mode → verify table background, text, borders, and hover states all properly invert
• Delete session_history.json while app running → app handles gracefully, no crash