feat(brand): use the Flagward mark in the nav, auth pages, favicon and app icons

[basb7]

What does this change?

The dashboard nav wore a generic foreground square with a flag glyph — a placeholder, not a brand. Now it wears the Flagward mark, and the auth pages do too.

• Nav: dashboard-nav.tsx renders /logo.png via next/image (priority, h-6 w-auto) instead of the bg-foreground square. The alt is deliberately empty: the link already reads as the brand name from the text beside it, so a labelled mark would have a screen reader announce the name twice.
• Auth pages: login and register center the same mark above the card (h-12). Same decorative-alt reasoning — each card title already reads "Flagward" (login.title, register.title), so the mark carries empty alt.
• App icons: new src/app/icon.png (512) and src/app/apple-icon.png (180), so Next serves /icon.png and /apple-icon.png by file convention — the build output confirms both routes.
• Favicon: favicon.ico regenerated from the same mark (25 KB → 5.7 KB, 16/32/48).
• Cleanup: the five unused Next.js template SVGs (file, globe, next, vercel, window) are gone; nothing referenced them.
• Small hygiene commit alongside: .DS_Store ignored.

frontend/brand/logo-source.png (the 1254px source the derivatives were cut from) stays out of the tree deliberately — it lives on disk if we need to re-cut, but this PR ships only what the product serves.

How was it verified?

ruff check .    All checks passed
pytest          616 passed
npm run lint    clean (biome, 69 files)
npm run build   compiled successfully (○ /icon.png, ○ /apple-icon.png)
npx vitest run src/app/register/page.test.tsx   6 passed

Backend is untouched; those two ran to confirm it. The SVG deletions were verified with a repo-wide grep for each filename before removal — zero references. The register suite passing with next/image in the tree confirms the mark renders without breaking the page's queries.

Checklist

• Tests added or updated for this change
• ruff check . passes
• pytest passes
• npm run lint and npm run build pass, if the frontend changed
• Documentation updated, if behaviour changed
• Commits follow Conventional Commits

Tests box left unchecked deliberately: a logo placement has no behaviour to pin — asserting an Image src would test implementation, not behaviour. Docs box unchecked: no documented behaviour changed.