feat(portal): one shared app bar on every portal app

[dwolfson]

Why

The back-to-Portal link was hand-rolled per page: top-left on some, top-right on others, shown only after choosing a persona on Catalog, Explorer and Lineage, and missing on Governance Metrics, Demo Admin and the docs viewer. Only Explorer and Data Mesh had a light/dark toggle, and identity badges had three different looks in three different places.

What

Every app now renders the same bar:
⌂ Portal | icon Title subtitle | page controls | extras · Promise/Memento · identity · ☀/☾

• static/app-bar.js (new, no framework):
  • Keeps the theme in one egeria-theme key and applies it in <head> before first paint.
  • Provides a declarative <header class="eg-appbar"> for plain-JS pages.
  • Adds a floating Feedback button for pages without React.
• AppHeader / ThemeToggle / useEgeriaTheme / ForLineageToggle in egeria-shared-ui.js for the React SPAs. The Promise/Memento toggle moves into the bar; it used to sit on top of the Feedback button.
• Portal link: always shown on every app.
• Identity: persona → 🎭; otherwise signed-in user → 👤; otherwise "Choose persona" (quickstart) or nothing (freshstart).
• Light theme:
  • :root.light palettes added to every page that lacked one.
  • Overview, Local Dashboards, Governance Metrics and the docs viewer re-theme their charts and diagrams when toggled.
  • The Portal keeps its own header but gains the toggle.
• Feedback: added to Lineage, Data Mesh, Local Dashboards, Governance Metrics and the docs. Overview's private copy is replaced by the shared one.
• tests/test_app_bar_consistency.py: stops pages drifting back to their own Portal link or theme handling (74 cases).
• Scope: quickstart, freshstart and portal-docs.
• BACKLOG.md:
  • Follow-ups: my-egeria TUI (needs pyegeria), Explorer vega charts in light mode, Data Mesh graph palette.
  • A separate investigation note on DigitalProductFamily Read refusals.

Testing

• Quickstart: checked in Chrome on every app, in dark and light: header layout, identity, toggle persistence across pages, Promise/Memento in the bar, no new console errors.
• Freshstart: wasn't running, so only syntax checks and the pytest guard.

Notes

egeria-overview.html is also changed by #609, #611 and #612. The hunks are in different places (header, feedback widget, vega config, user chip), but whichever merges later may need a rebase.

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