web(timeline): thin the three timeline pages onto a shared TimelineGate; host-cover TimelineState transitions

[mdorman]

Boy-scout follow-up: #653/#669 converged the tag pages but touched SiteTagPage,
which still carries #[expect(clippy::too_many_lines)]. Per the #306 thin-component
direction, decompose the three timeline pages — and, following the #655 example
(the analogous TagInput thinning, #328), do it so the decomposition turns
currently-exempt reactive code into host-tested code, rather than merely
shuffling wasm sub-components.

The three pages

SiteTagPage, UserTagPage, and UserTimelinePage (web/src/posts/component.rs)
are now byte-for-byte near-identical wiring around the shared crate::timeline
bundle: build a keyed Resource, TimelineState::default() + a loaded gate,
adopt the PageSeed seed, a resolve Effect, a spawn_load_more load-more, a
read_error Memo, then chrome + the error/loading/TimelineRows gate. All three
carry #[expect(clippy::too_many_lines)].

What to do (mirroring #655)

• Host-testable core. #329 already split the pure model
  (TimelineCursor/LoadStatus/fold logic → host-tested timeline/state.rs) from
  the reactive TimelineState signal-bundle (wasm-only timeline/component.rs,
  gated at mod.rs:14). The one uncovered behavior is TimelineState's
  transitions (adopt/resolve/fail, and the page's "apply the fetched result"
  dispatch) — exempt today only because the bundle is wasm-gated. Relocate the
  TimelineState signal-bundle into host-compiled state.rs and cover its
  transitions under an Owner (the web::reactive::Invalidator / forms::Field
  convention web(tags): co-locate TagInput into the tags vertical (#328) #655 used) — no #[client_only]. This is the "decomposing made it
  host-testable" win.
• Shared reactive residue → one component + one helper. Extract a TimelineGate
#[component] (the error → loading → <TimelineRows/> gate, taking
  state/loaded/read_error/on_mutate/on_load_more + tag_context/empty_text)
  and a wire_timeline_resolve(state, loaded, initial_page) -> Memo<Option<String>>
  helper (the resolve Effect + the failure Memo). Both in crate::timeline.
• Sweep all three pages onto the extracted pieces; each collapses to its
  page-specific bits (params, keyed Resource, PageSeed adoption, load-more fetch,
  chrome). Drop all three #[expect(clippy::too_many_lines)].

Acceptance

• All three timeline pages free of #[expect(clippy::too_many_lines)]; no
  #[client_only] added.
• TimelineState host-compiled with Owner-based tests covering adopt/resolve/
  fail (+ the apply-result dispatch); coverage stays clean (the relocated code is
  now measured — the tests are what keep the gate green).
• TimelineGate + wire_timeline_resolve shared by all three pages (and consider
  home/cockpit, which already inline the gate differently — evaluate, don't force).
• cargo xtask validate green (incl. e2e); behavior identical — the tag/profile e2e
  (incl. posts: converge SiteTagPage/UserTagPage onto the shared TimelineState bundle #653's user tag page smoke test) stays green.

Origin: #653/#669 (touched SiteTagPage). Model: #655/#328. Family: #301, #306.