fix(#567,#568,#561,#562): button size prop, navbar active link, dynamic imports, SEP-0010 auth

[marshall8000]

#567 — Button size prop (sm/md/lg)

• Button component already had complete size prop implementation
• Verified: size prop typed as 'sm' | 'md' | 'lg' with default 'md'
• Each size maps to distinct Tailwind padding and text-size classes
• All existing usages without size prop continue to render identically

#568 — Navbar active link with prefix matching

• Updated isActive() to use pathname.startsWith() for prefix matching
• Added /invoice nav item so /invoice/* routes highlight correctly
• Exact match preserved for root-level routes
• No link highlighted on pages that don't match any nav item

#561 — Dynamic imports and route-level code splitting

• NotesEditor: added NotesEditor.dynamic.tsx with next/dynamic + ssr:false
• TrendChart: created new TrendChart component + TrendChart.dynamic.tsx
• transactionBuilder: new src/lib/stellar/transactionBuilder.ts loaded dynamically in invoice sign page to exclude stellar-sdk from initial bundle
• next.config.js: fixed bundle analyzer (ANALYZE=true gates report), fixed withPWA config, fixed syntax error, fixed env var typo

#562 — SEP-0010 Web Authentication

• GET /api/auth/challenge: returns signed challenge tx (5 min TTL)
• POST /api/auth/token: verifies signed tx, issues 24h JWT in httpOnly cookie
• src/lib/auth/withAuth.ts: route handler wrapper requiring valid JWT (401 if absent/expired)
• src/hooks/useSep10Auth.ts: challenge->sign->token flow hook with auto-retry on 401
• WalletContext: extended with sep10 auth state, auto-authenticates on wallet connect

Misc: cleaned up .gitignore (removed duplicates, added all test artifact patterns)

closes #567
closes #568
closes #561
closes #562

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