fix(web): make the beads view inherit the SourceHut theme

The beads board shipped its own Mardi Gras palette — a purple→green
marquee gradient and saturated solid lane fills — that clashed with
SourceHut's flat, utilitarian dark theme, and the detail pane's field
labels were a hardcoded dark purple with no dark-mode override, leaving
"Created by / Owner / Description" unreadable on the dark background.

Drive everything from CSS variables that mirror core.sr.ht's Bootstrap
palette for both the light default and the prefers-color-scheme: dark
variant sourcehut ships (the same media query it uses), so the view
inherits the host theme instead of fighting it. Lane hues become muted
Mardi Gras accents (gold/green/violet/gray) used only as thin borders and
low-alpha tints — never as body text — so contrast holds on either theme.
The marquee is now a flat stat row, lanes an understated panel with an
accent underline, and detail labels a readable muted caption. Verified
light and dark via a headless render.

feat(web): Mardi Gras beads view

Render a beads (bd) issue database as a Mardi Gras parade board: four
lanes — Rolling (in progress), Lined Up (open & ready), Stalled (open &
blocked), Past Stand (done) — plus a marquee of per-lane counts. Cards
show id/title/type/priority/assignee/labels and blocked-by/blocks counts;
an issue-detail pane (?issue=<id>) shows the full issue with its
dependency edges in both directions and its comments thread.

Fingerprints on the issues + dependencies tables and reads everything
through the browse layer (column-name-keyed, so column order is
irrelevant), capped at 2000 rows/table. Status is bucketed via
custom_statuses with name heuristics as a fallback; "blocked" means
is_blocked or an open blocking dependency. Plugs into the view registry
via init()+RegisterView with no framework edits. Purple/gold/green
palette scoped in the template (no scss rebuild needed).