feat(web): promote header to global app shell

[Willxup]

Summary

• Promote the global header outside <main> so the app shell owns the top-level layout and the header stays above page range controls.
• Apply the approved compact C design with a 54px desktop header, revised FlowLens mark, tighter actions, and network telemetry labeling.
• Introduce semantic layer tokens so page controls remain below the header while dialogs and tooltips remain above it, with 44px mobile action targets.

Verification

• make check
• make frontend-e2e when UI behavior changes
• Public API documentation updated when applicable
• No real configuration, credential, Cookie, database, backup, log, or private address added
• Commits include Signed-off-by (DCO)

Compatibility

No API, configuration, database, or container changes. The header is now a sibling of <main>, and .app-shell owns data-source-mode; bundled styles and tests are updated together.