fix(frontend): make ticker logos exchange-aware (#623)

[AtomicTrxn]

Description

Root cause:

• Asset avatars currently resolve logos with a simple fullSymbol -> baseSymbol lookup against /ticker-logos/*.png.
• For ambiguous tickers like DTE, that means a German/Xetra asset with symbol DTE falls back to DTE.png, which is the US listing logo.
• The rest of the app already carries exchange context (exchangeMic), but the logo lookup path was ignoring it.

Lookup method in this PR:

• If the symbol is already exchange-qualified, keep it unchanged.
• If the symbol is unsuffixed and the view has an exchangeMic, derive the Yahoo-style suffix from that MIC (for example XETR -> .DE, XFRA -> .F, XLON -> .L) and try the exchange-qualified logo first.
• Only allow the unsuffixed base-symbol fallback when the exchange is unknown or canonically unsuffixed (US-style cases).
• If a non-US exchange-qualified logo asset is missing, fall back to initials instead of showing an incorrect logo.

Fix summary:

• Made TickerAvatar exchange-aware in both frontend/shared components.
• Propagated exchangeMic through asset, activity, holdings, dashboard, and allocation avatar call sites.
• Added optional exchangeMic to holdings Instrument and allocation HoldingSummary payloads and populated them in core services.
• Added regressions for the avatar lookup, UI wiring, and core payload propagation.

User-visible tradeoff:

• Some non-US assets may now show initials instead of borrowing an incorrect unsuffixed US logo when no exchange-specific logo asset exists. This is intentional to prefer no logo over a wrong-company logo.

Validation:

• pnpm --filter frontend test -- ticker-avatar
• pnpm --filter frontend test -- top-holdings
• cargo test -p wealthfolio-core exchange_mic
• cargo test -p wealthfolio-core portfolio::holdings
• cargo test -p wealthfolio-core allocation_service
• pnpm --filter frontend lint (passes with existing repo warning baseline)
• cargo fmt --all -- --check
• pnpm exec prettier --check ... on all touched frontend/package files
• pnpm --filter frontend type-check still fails on pre-existing unrelated issues in apps/frontend/src/addons/type-bridge.ts and apps/frontend/src/pages/holdings/components/alternative-holdings-table.tsx

Fixes #623

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