fix(docs): force ER attribute text color via themeCSS in dark mode

[pratyush618]

Follow-up to #131. The 7-slide schema carousel showed alternating dim text in dark mode — odd rows ended up with washed-out characters that were nearly unreadable.

Root cause

Two compounding issues:

1. Mermaid's auto-contrast for ER rows. With odd-vs-even row backgrounds set to two slightly different shades (#1f2024 / #272a30), Mermaid's internal heuristics chose different text colors for each — and in dark mode the choice for one of them landed on a low-contrast grey instead of the configured textColor.

2. themeVariables.textColor doesn't propagate to ER .attribute-text reliably. The ER renderer uses its own internal class hierarchy (.er.attributeBoxOdd, .er.attributeBoxEven, .er .attribute-text) and doesn't always read from textColor. Some Mermaid versions ship hardcoded fills.

Fix

Three layers of defense in docs/src/components/mermaid.tsx:

1. Drop the zebra stripe — set both attributeBackgroundColorOdd and attributeBackgroundColorEven to the same value (#1f2024 dark / #ffffff light). Removes the auto-contrast asymmetry entirely.
2. darkMode: true flag in dark theme variables. Tells Mermaid which way internal heuristics should lean for any color it auto-picks.
3. themeCSS override with !important — explicit CSS injected into every Mermaid SVG forcing .er.entityBox, .er.entityLabel, .er.attributeBoxOdd/Even, .er .attribute-text, and .er text to the right fill colors. Belt-and-suspenders against any Mermaid-version-specific hardcoded values.

Light theme keeps a subtle alternation (#ffffff / #f4f4f5) since the contrast is fine on a light bg.

Test plan

• pnpm --dir docs types:check clean
• pnpm --dir docs lint clean
• pnpm --dir docs build clean
• CI: Deploy Docs job
• Visual: open /docs/architecture/storage in dark mode, flip through all 7 carousel slides — every column (TYPE / NAME / KEY) should be uniformly readable on every row.
• Light mode same page — subtle row striping, all text readable.
• Other Mermaid diagrams still render correctly (/docs/architecture/job-lifecycle, /docs/architecture/scheduler).