fix(control-center): make Growth language-first

[i-xtsu-sixyou-ken-mei]

Intent

Make the Growth tab reflect the actual social publishing contract: one article is released to every active platform together, while language is the only lane-level variable.

Context

The current Growth UI still exposes stale per-platform content recommendations such as Prioritize <topic>, title-shaping advice, and Rednote-specific hashtag guidance. That conflicts with the current shared release cohort and language-rotation model.

LatestEpisodePanel also renders a bare metric number without saying it is views, hardcodes 24h even when the response window is latest, and loses a published platform entirely when that platform has no usable metric snapshot.

Scope

• Replace topic/title/hashtag prioritization UI with language-performance cards.
• Redesign the publishing summary around the three shared JST article slots.
• Make the latest-episode panel explain its metric window and label views explicitly.
• Link each latest-episode platform to its persisted post_url.
• Keep published lanes visible when telemetry is missing and render them as a collection gap instead of zero.
• Prefer the zh-Hant published title for the shared episode summary.
• Add regression coverage for the language-first UI, post links, missing metrics, and zh-Hant title preference.

Out of scope

• Changing the social release schedule or daily cap.
• Changing language-rotation assignment.
• Removing historical topic/packaging telemetry from the backend learning model.
• Changing metric collectors for Rednote or YouTube.

Product contract / invariants

• This PR does not change a documented product/architecture invariant.
• This PR intentionally changes an invariant with explicit product approval.

Affected invariant:

apps/podcast-pipeline/AGENTS.md / social release contract: each episode releases as one cross-platform cohort; language coverage/rotation remains the lane-level variation.

Why:

The UI is being aligned with the existing release invariant rather than presenting obsolete per-platform content decisions.

Acceptance criteria

• Growth no longer renders Prioritize <topic>, Shape the title like this, or What to publish next.
• Growth presents language performance as the actionable comparison.
• The publishing summary shows 09:30 / 12:00 / 16:00 JST as shared article slots.
• Latest episode shows an explicit metric window and views label.
• Published platform links remain available even when metrics are unavailable.
• Missing metrics are represented as unavailable telemetry, not zero views.
• Shared episode title prefers zh-Hant when a zh-Hant social post exists.

Implementation

• GrowthView.tsx now renders PublishingCadence, LanguagePerformanceCard, and a window-aware LatestEpisodePanel.
• Language leaders are ranked by follower conversion first, then median 24h reach/sample count as fallback evidence.
• buildEpisodes() now builds summaries from published posts even when no usable metric exists, preserving postUrl with null metric fields.
• buildEpisodes() tracks language_code and replaces a fallback title with the zh-Hant published title when available.

Contract alignment

• Implementation matches the invariant.
• Regression / contract tests cover it.
• Scoped AGENTS.md remains accurate.
• README / runbook remains accurate.

Validation

• GitHub branch diff review — PASS; four intended Control Center files changed and branch is based directly on current main.
• Social release contract slot constant remains 09:30, 12:00, 16:00, so the existing repository contract check continues to compare the same values.

Validation gaps

Executable test/type-check commands could not be run in the connector-only environment. CI should run the repository gates on this PR.

Known unrelated failures

None known.

Reviewer notes

Please scrutinize the buildEpisodes() behavior change: a published lane with unavailable/no standardized metric now intentionally survives into the episode summary with null metric fields so the UI can expose its post link and collection gap.