[WEB-002] Capture real app UI with privacy-safe demo data

[veryCrunchy]

Goal

Display accurate mobile and desktop product screenshots generated by the real Compose application while guaranteeing that no account, endpoint, credential, cache, contact, message, file name, or personal media can enter the artifacts.

Requirements

• Add a deterministic demo mode backed by synthetic platform services and committed non-personal fixtures.
• Render the actual production navigation, screens, components, typography, responsive breakpoints, and theme tokens.
• Capture separate Android phone and desktop window layouts at controlled dimensions and pixel density.
• Cover representative Files, Photos, Talk, sync, and adaptive-app experiences without hand-redrawing the UI.
• Keep demo mode structurally isolated from saved sessions, network clients, platform media libraries, home directories, and production caches.
• Make screenshot output deterministic enough for review and visual regression checks.
• Add automated guards that reject credentials, URLs, email-like values, absolute user paths, content URIs, and fixture/output drift.
• Let the website crop or frame captures without altering the product UI itself.
• Mark or remove temporary hand-rendered mock screenshots once real captures replace them.

Acceptance criteria

• Website mobile images come from the actual Android app UI running only synthetic data.
• Website desktop images come from the actual desktop app UI running only synthetic data.
• A clean checkout can reproduce all approved captures without access to a Nextcloud account or private environment.
• Capture tests fail if real platform services, sessions, endpoints, user storage, or nondeterministic fixtures are accessed.
• Screenshots visibly match the corresponding application build and can serve as visual-regression baselines.