Frontend half of #607 v0 — adds the `◆ M4TR1X ◆ →` page-link entry
to the dashboard tab strip between SCH3DUL3S and FL0W →. Same
`tab-link` class as FL0W → since /matrix/ is its own SPA, not an
in-place pane swap.
Hidden via the `hidden` attribute by default; tabs.js's `refreshState`
flips it based on `s.matrix_gui_enabled` (added in #610). When the
operator hasn't set `hyperhive.matrix.gui.enable = true` the snapshot
returns false and the tab strip doesn't surface a dead link.
docs/web-ui.md updated:
- chrome-header tab-strip list now includes M4TR1X → with the
hidden-when-disabled note
- new "M4TR1X page" section under FL0W explaining the same-origin
serve via hive-c0re ServeDir + the manual homeserver URL pick on
first login (with #609 tracking the post-#15 nginx-front re-root +
.well-known/matrix/client auto-discovery)
40b9a7f2cd