fix: remove duplicate voting navigation controls

[Findeton]

Parent issue: https://github.com/sequentech/meta/issues/13189

Replace nested link/button navigation controls with one MUI-styled router link across Start, Vote, Review (including errors), Audit and Ballot Locator. Keep disabled Start as a disabled native button and preserve destinations, query strings and CSS hooks.

Verification: all 70 voting-portal Jest tests pass (including the new failing-before/passing-after navigation regression); touched-file ESLint and formatting pass. Browser demo checks cover forward/reverse Tab, Enter, Start gating, contest Back, Edit ballot and locator Back. No real vote cast. Actual screen-reader speech and Audit/result-only states remain manual checks. Full TypeScript validation is blocked by existing dependency/portal errors; no diagnostics point to changed files.

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