New squad export theme: "Chalkboard" (tactical blackboard)

[tobiager]

Context

Repo Squad exports currently support two themes, floodlight (dark, green accents) and grass (green pitch,
gold accents) — defined in two places that must stay in sync by hand (see Findings in
docs/exports.md):

• app/api/og/_shared/squadTheme.ts (SQUAD_THEMES, used by the Satori
  PNG routes).
• palette() in lib/svg-squad/render.ts (used by the hand-built SVG route).

The ask: add a third theme, "chalkboard" — a tactics-board look (dark board background, white/chalk-colored
pitch lines and text, like a coach's whiteboard diagram) as a third selectable option everywhere floodlight/
grass are selectable today.

Files involved

• app/api/og/_shared/squadTheme.ts — add a chalkboard entry to SquadThemeName and SQUAD_THEMES.
• lib/svg-squad/render.ts — add the matching case to palette().
• components/squad/SquadExportPanel.tsx — THEME_META record and the theme selector UI (Theme type,
  currently "floodlight" | "grass").
• app/api/og/squad/[owner]/[repo]/route.tsx and app/api/svg/squad/[owner]/[repo]/route.ts —
  parseTheme()/theme query parsing (currently value === "grass" ? "grass" : "floodlight" — needs a third branch).

Acceptance criteria

• ?theme=chalkboard works on both the PNG route (/api/og/squad/...) and the SVG route
  (/api/svg/squad/...), producing a visually coherent "coach's blackboard" look on both.
• The new theme is selectable in SquadExportPanel's theme pills, alongside Floodlight/Grass.
• Falls back sanely (to floodlight) on an unrecognized ?theme= value, same as today.
• Colors/contrast are checked against both the portrait and landscape formats — a palette that reads fine on
  the tall portrait pitch can still clip/clash on the short 630px landscape header (see the sizing constants'
  comments in the OG route for why landscape is the tight case).

Notes

Reuse mowStripes() in squadTheme.ts if the board look benefits from a subtle texture — but a flat background is
also a perfectly fine v1.