feat: Add bulk position management and batch transaction building

[OTimileyin]

Summary

This pull request implements the Bulk Position Management feature (Issue #15). It introduces multi-select checkboxes to the Overview/Dashboard position list, allowing users to batch multiple actions into a single signed transaction.

Features implemented:

• Checkbox Table Columns: A Select-All checkbox in the table header, and row-level checkboxes. Clicking the checkbox toggles state without triggering row detail navigation.
• Shift-Click Multi-Select: Standard browser range selection via Shift + Click.
• Floating Bulk Actions Bar: Appears dynamically at the bottom of the dashboard when 1 or more positions are selected. Shows selected count, Close Selected button, and Adjust Leverage inputs.
• Confirmation Modal: Displays affected positions, leverage changes, and estimated health factors with color-coded safety indicators before signing.
• Smart Transaction Batching: Multi-asset actions targeting the same Blend pool are bundled into a single contract call (submit_with_allowance). Multiple pools are added as separate operations in a single Stellar transaction (requiring only one wallet signature).
• Safety Limits: Dynamically checks and intercepts leverage target adjustments that exceed the asset's safe limits.

Related Issue

Closes #15

Checks

• I read the contribution guide.
• I kept this pull request scoped to the linked issue.
• I ran the relevant local checks or explained why they were skipped.
• For Drips wave issues, I claimed the issue before opening this pull request.

Notes for Reviewers

• Transaction Building & Security: Safe limits are validated at calculation time. We've written a custom Vitest unit test suite located at frontend/test/bulk_tx.test.ts to mock the Stellar SDK and verify XDR construction for both Close and Adjust paths.
• Demo Mode Testing: For local testing convenience without active testnet/mainnet wallets, clicking "View Demo" on the landing page now populates the Dashboard with multiple mock positions so the multi-select range selection and confirmation modals can be interacted with directly.

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