refactor(home): restructure the feed pipeline, fix pagination stalls, and add a cards-per-row setting

[A-EDev]

Summary

Splits the 2,166-line home view model and the 651-line screen into focused files, moves the feed's pure logic out where it can be tested, and puts seed and related-graph retrieval behind an injected collaborator. On top of that it fixes the feed silently refusing to paginate, and adds a cards-per-row setting for the home grid.

Related issue

Closes #765
Closes #855
Closes #925

Change type

• Bug fix
• Feature
• Refactor or maintenance
• Build, packaging, or CI
• Documentation

Validation

• ./gradlew :app:assembleGithubDebug
• ./gradlew :app:testGithubDebugUnitTest
• I ran any additional flavor-specific build or test tasks affected by this change.
• I manually tested the affected behavior on an Android device or emulator.

Also run and passing: ./gradlew spotlessCheck, :app:compileGithubDebugKotlin, :app:compileFossDebugKotlin.

Test device and Android version: Xiaomi POCO X6 Pro, HyperOS 3.0.9.0, Android 16

The refactor commits are intended to be output-identical; the two visual changes and the two
behaviour changes are called out below. No performance claim is made anywhere in this PR — the
structural changes were not profiled.

Screenshots or recordings

Risk and compatibility

Fixes

• Homefeed scroll bug #765 — the home feed stopped paginating and only recovered if you scrolled up and back down.
  A load-more page that appended nothing (every candidate already on screen) set allowRestart = false
  and gave up. Nothing was appended, so the feed length was unchanged, so the last-visible index
  could not change, so the index-only viewport flow had nothing to re-emit — the queue could never
  re-arm. hasMorePages is never set false on the Flow-feed path either, so the end-of-feed footer
  did not show: the page just ended silently. Two fixes: an empty page now retries up to 3 times
  with backoff (each attempt advances the discovery-query cursor and rotates seeds, so it asks for
  different content), and the viewport flow now emits on isScrollInProgress as well as the index,
  so a further scroll attempt re-arms it.
• [Feature]: Home Layout in Multiple Grid mode #855 / [Feature]: Flow UI #925 — no way to get more than one card per row. New Cards per row setting
  (Auto / 1 / 2 / 3) under Settings → Content display → Home layout, shown only in Grid mode since
  List is one per row by definition. Auto is the default and keeps today's behaviour.
• A large-screen breakpoint gap. A ~600dp screen fell into the <700dp bucket and got a single
  full-width poster. Auto now gives 2 columns from 600dp up; below 600dp and every wider breakpoint
  are unchanged.
• A per-item LaunchedEffect in the feed grid called enrichChannelMetadataIfMissing(videoId),
  whose first line was a linear scan of the whole video list — re-running for every item on every
  scroll recycle. It now takes the Video the call site already holds.
• The feed item block was written twice (once before the shelves, once after) and the second copy
  had silently lost its home_video_card test tag, so instrumentation only ever saw the first
  1–4 cards.
• Dead code in FlowNavigation: an inlinePlayerHeight that read two per-frame player animation
  values and was never used anywhere.
• A redundant second viewModel.initialize(context) in HomeScreen — FlowApp already calls it
  with the application context before the NavHost composes.

Behaviour changes worth knowing about

• uiState moved from SharingStarted.Eagerly to WhileSubscribed(5_000). The dedup pass over
  the feed no longer runs on every background append while Home is off-screen.
• Seed building no longer races app start. The view model held two fields pointing at the same
  ViewHistory singleton — a lazy one and a nullable one set in initialize(). Seed building used
  the nullable one and returned an empty list when it was still null, which on a cold start could
  leave the related lane empty because init starts loadFlowFeed before FlowApp calls
  initialize(). With constructor injection it is never null, so cold-start related seeds now
  populate.
• The Shorts shelf position rounds up to a multiple of the column count. It spans every column,
  so at 3 columns the old fixed index would have started it after one card and left holes in the row
  above. Unchanged at every Auto breakpoint (1 / 2 / 3 / 4).
• Two shared state components adopted on Home — FlowEmptyState for the feed-disabled state and
  FlowErrorState for the error state. These look different from the local versions they replace:
  a larger icon container, bodyLarge title, and a filled button instead of a text button. The
  retry label is unchanged (retry and action_retry are both "Retry").
• The load-more spinner uses the existing shared FlowFeedProgress instead of a bare
  CircularProgressIndicator.
• ContinueWatchingShelf moved out of ui/components/VideoCard.kt into a new
  ui/components/home/, along with its two private helpers. It had exactly one consumer.
• Color.Red in the Shorts shelf is now a named theme token wired to the existing YouTubeRed
  literal. Color.Red is 0xFFFF0000, so this is pixel-identical.
• New HomeFeedCacheRepository Hilt provider, matching the existing @Provides @Singleton
  delegate pattern. Four getInstance() / manual-construction sites in the view model are gone;
  instance identity is preserved.
• New preference home_feed_columns, default AUTO. DataStore only — no Room schema change.
• 3 new strings in values/strings.xml. Other locales are left to Weblate.
• The Continue Watching tap builds its Video with a local helper rather than the shared
  VideoHistoryEntry.toVideo() mapper: that one carries isShort, which FlowNavigation branches
  on, so reusing it would have rerouted a resumed short to the Shorts player.

Known limitation

HomeViewModel.kt is 2,166 → ~1,270 lines, still over the 600-line ceiling. Everything that
separates cleanly has been extracted; what remains is one responsibility — loading and paging the
feed. Splitting it further would put the shared discovery-query cursor (discoveryQueries +
currentQueryIndex, advanced by wave 1, wave 2 and load-more alike) across a class boundary, which
is worse than a long file. Moving that cursor into its own owner is the honest prerequisite and is
left for a follow-up.

The cards-per-row setting is scoped to the home feed, so at 600–700dp Home shows 2 columns while
Subscriptions still shows 1. Extending it is a follow-up.

• The change does not introduce secrets, private data, or unexpected telemetry.
• New user-facing text uses Android string resources.
• Dependency and lockfile changes are intentional and limited to this PR.
• Room schema changes include the required version bump and migration, or this PR does not change the Room schema.
• Breaking changes and upgrade steps are clearly documented.

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