feat: add e-ink mode setting with tap-to-scroll

[dalekirkwood]

Summary

Adds a manual e-ink mode toggle under UI Settings. When enabled:

• Tap zones in the reader: tap the bottom half of the screen to page down one viewport, tap the top half to page up. Uses non-animated scrollTo for clean e-ink refreshes (no ghosting from animation frames).
• Drag-scrolling disabled in the reader. Only tap-to-page works, which is the standard e-reader paradigm.
• Navigation transitions disabled: instant screen changes instead of fade animations.
• Header image crossfade disabled: crisp image load, no fade ghosting.
• B&W color scheme: pure black on white for all app chrome (toolbar, cards, FAB, drawer). Distinct gray steps that e-ink panels can actually distinguish.
• E-ink article template: pure black text on white, font-weight: 500 for thicker rendering, black underlined links, no colored elements.

The setting is off by default: existing users see zero behavior change.

Implementation

Follows the existing scrollToProgressEnabled toggle pattern:

• SettingsDataStore + SettingsDataStoreImpl: new eInkMode pref (SharedPreferences-backed, with reactive StateFlow)
• UiSettingsViewModel / UiSettingsScreen: new Switch row
• BookmarkDetailScreen: invisible overlay Box on top of the content intercepts taps (WebView consumes touches, so the overlay must sit above it); verticalScroll(scrollState, enabled = !eInkMode) disables drag-scrolling
• MainActivity: conditional NavHost transitions; ReadeckAppTheme accepts eInkMode to switch color scheme
• html_template_eink.html: new Sakura.css variant stripped of all color
• BookmarkDetailViewModel: template flow switches to e-ink template when mode is on

Known limitations

• Site-name click in reader does not work in e-ink mode. The tap-zone overlay covers the full screen, so the clickable site-name row (which opens the article URL in a browser) is intercepted. The FAB menu and back button are unaffected.
• BookmarkDetailViewModel reads eInkMode once in init (non-reactive), consistent with the existing scrollToProgressEnabled pattern. Toggling the setting while reading an article takes effect on next article open.

Test plan

• ./gradlew compileGithubReleaseDebugKotlin passes
• ./gradlew testGithubReleaseDebugUnitTest all tests pass
• Manual: toggle e-ink mode on, nav transitions are instant
• Manual: open an article, tap bottom half pages down, tap top half pages up
• Manual: e-ink mode off is identical to current behavior
• Manual: B&W color scheme applied to list, settings, and reader
• Manual: article content is pure black on white with thicker text
• Manual: drag-scrolling is disabled in the reader when e-ink mode is on

Files changed

14 files, +427/-42 lines. No new modules, no new dependencies.