Align vote URL display length with SV IDs

[loglapa]

Summary

• let CopyableUrl accept a caller-specific visible content length while preserving its existing default
• render vote-reason URLs with the same typography and total character count as the corresponding SV ID
• keep the sanitized full URL in the link target and clipboard value
• add a focused proposal-details regression test

Closes #6914

Validation

• reused Daml and OpenAPI TypeScript artifacts generated with the pinned DPM SDK 3.5.2 and OpenAPI Generator 6.6.0
• npm run test:sbt -w @canton-network/splice-sv-frontend -- src/__tests__/governance/proposal-details-content.test.tsx (32 tests)
• npm run check -w @canton-network/splice-sv-frontend
• npm run type:check -w @canton-network/splice-sv-frontend
• npm run build -w @canton-network/splice-sv-frontend

[loglapa]

Rebased this on the current main after the proposal-details scroll/layout refactor in #6410. The update now preserves the new horizontally scrollable URL behavior and makes vote URLs fill the same visual slot as SV IDs, rather than restoring the obsolete character truncation. Validation on the rebased head: workspace formatting, ESLint, TypeScript type-check, and all 31 focused proposal-details tests pass.

[loglapa]

Rebased onto current main (356924ab) and resolved the overlap with #6955 by keeping the new upstream CopyableUrl implementation unchanged. This PR now only opts vote URLs into upstream's fullWidth behavior and retains the focused layout assertions. Post-rebase Prettier and ESLint pass; the branch is one clean DCO-signed commit with the personal GitHub identity. Full local typecheck requires generated DAML/OpenAPI artifacts unavailable in this fresh worktree; the focused proposal-details tests passed before the base-only update.