feat: add Kindle-style reading time tracker

[BenjaminMichaelis]

Summary

Adds a Kindle-style "time left in chapter / book" indicator to the EssentialCSharp.Web header, adapting to each user's actual reading speed over time.

Features

Server-side

• WordCountService — singleton that walks all SiteMapping entries at startup, strips <pre>/<code>/<script>/<style> via HtmlAgilityPack, and caches per-page/per-chapter/book-wide prose word counts + prefix sums (O(1) lookups at request time)
• ReadingActivity + UserReadingProfile — EF Core entities (migration AddReadingTimeTracking); profile stores {totalWords, totalActiveSeconds} for O(1) WPM derivation; activity rows capped at 500/user for bounded storage
• ReadingController — four endpoints:
  • POST /api/reading/session — batch interval ingestion, WPM algorithm (>900 WPM hard-discard; slow outliers clamped at 0.25 × referenceWpm), aggregate update + retention trim
  • GET /api/reading/profile — current WPM for logged-in user
  • GET /api/reading/book-stats — public word counts per chapter + total
  • POST /api/reading/reset — clears aggregate + detail rows

Client-side

• useReadingTracker.js composable — activity state machine (READING / IDLE / HIDDEN), 5-min idle threshold, instant-pause on visibilitychange, flush-on-navigate, localStorage→server sync on first authenticated page load
• ReadingTimeRemaining.vue — displays "X min left in chapter · Y min left in book" in the header; recomputes every few seconds from live scroll fraction; only renders on chapter pages; integrated into HeaderStatus.vue
• window.* globals — CURRENT_PAGE_KEY, PAGE_WORD_COUNT, CHAPTER_WORD_COUNT, WORDS_BEFORE_PAGE, CHAPTER_START_WORDS, BOOK_WORD_COUNT emitted from _Layout.cshtml

UX

• Reset reading speed button added to Identity → Manage Profile page

Algorithm

Mirrors Kindle's ReadingTimer:

• Upper outlier: intervals >900 WPM are discarded (page-flip/skim)
• Lower outlier: intervals slower than 0.25 × referenceWpm are time-clamped (walked away mid-page)
• Aggregate: totalWords / (totalActiveSeconds / 60) — never stores a running average that needs re-weighting
• Cold start: 200 WPM display default (not persisted) until first accepted interval

Testing

• WordCountServiceTests: prose-only counting, <pre>/<code> exclusion, prefix sums, chapter/book totals
• ReadingControllerTests: auth guards (401 on all mutating endpoints for anonymous users), WPM algorithm constant verification, GET /api/reading/book-stats shape test