fix(feed): round-2 deep-review remediations (incl. a real coerce-on-seed bug)

The re-review of the fix round surfaced 4 findings; fixing the coverage one
exposed that the C1 migration was ineffective:
- FeedFiltersProvider.loadAccountFilters: run coerceTagModes on the RAW stored
  blob, not the object merged over DEFAULT_FILTERS. The merge injects
  DEFAULT_TAG_MODES (all "or"), which coerceTagModes prefers over the legacy
  scalar `tagMode` — so the migration never fired. Now a persisted legacy
  tagMode:'and' actually migrates into tagModes.topic (proven by a new test).
- test_feed_views: the tag_mode_other=and case was vacuous (only chC carried
  either "other" tag, so AND == OR). Give chA an "other" tag too so OR-other
  {chA,chC} differs from AND-other {chC} — the conjunctive path now narrows.
- DateFilterSelect: reset the custom reveal in useLayoutEffect (not useEffect)
  so the correction commits before paint — no one-frame "Custom range" flicker
  on the trigger after an external clear.
- SavedViewsProvider: extract the sticky-view tie-break to a pure, tested
  lib/savedViews.ts (preferSticky).

New tests: savedViews (preferSticky), FeedFiltersProvider.dom (legacy tagMode
migration on the localStorage/default-view seed).

backend/tests/test_feed_views.py

@@ -105,11 +105,12 @@ def _seed_tagged(db, make_user):
     chans = {c: Channel(id=c, title=c, subscriber_count=10) for c in ("chA", "chB", "chC", "chD")}
     db.add_all(chans.values())
     db.flush()
-    # channel -> (system) tag keys. chA carries two languages (L1+L2) and chC two "other" tags
-    # (O1+O2) so the per-category any/all match narrows non-trivially for language and other, not
-    # only topic (chA/chB share topic tags T1/T2).
+    # channel -> (system) tag keys. chA carries two languages (L1+L2); chC two "other" tags (O1+O2)
+    # AND chA carries O1 too — so the per-category any/all match narrows NON-TRIVIALLY (AND != OR) for
+    # language and other, not only topic: e.g. other {O1,O2} → OR {chA,chC} vs AND {chC}; language
+    # {L1,L2} → OR {chA,chB,chC} vs AND {chA}. (chA/chB share topic tags T1/T2 for the topic case.)
     links = {
-        "chA": ["L1", "L2", "T1"],
+        "chA": ["L1", "L2", "T1", "O1"],
         "chB": ["L1", "T2"],
         "chC": ["L2", "T1", "O1", "O2"],
         "chD": [],