feat: add Community pod filter

[lilyshen0722]

Summary

• add an authenticated scope=community pod listing for non-personal publicRead pods while preserving the default membership-only listing
• add a bilingual Community/社区 sidebar tab backed by a separate, deduplicated discovery dataset so All/Team/Private remain personal
• exclude every DM/admin pod shape defensively and keep the four-tab row on one line at desktop and mobile widths

Proof

• backend controller + service suites: 24 tests passed
• full frontend suite: 61 suites / 270 tests passed
• i18n manifest, base-key parity, and CLDR completeness: passed
• backend + frontend production builds: passed
• changed frontend files: ESLint 0 errors
• Chromium: EN + zh-CN at 1280x900 and 390x844; one row, scrollWidth === clientWidth, no document overflow, no page errors

Privacy invariant

Community discovery is stored separately from the membership list. The regression tests assert that the same non-member public pod appears under Community and remains absent from the default/All view.