fix(sidebar): add native session links (#460)

[rodboev]

Summary

• Kept .session-item as the row container and moved the primary session surface to a native anchor using router.buildSessionHref(session.id).
• Kept row-level button discoverability and keyboard activation on the SPA session-selection path, while restoring normal row selection for the visible non-link parts of the sidebar item.
• Preserved nested controls (tree toggle, rename flow, star button, side meta) by not converting the whole row into a link.
• Added explicit Open in new tab to the sidebar session context menu, using the same canonical URL and window.open(..., "_blank", "noopener").

Scope

• Limited to frontend/src/lib/components/sidebar/SessionItem.svelte and frontend/src/lib/components/sidebar/SessionList.test.ts.
• Keeps the outer row container and existing nested controls intact; broader session-entry surfaces stay out of scope for this slice.

Review Notes

• No full-app navigation is added in this slice; both the anchor activation path and the row overflow click surface keep SPA state semantics via sessions.selectSession(session.id) while still exposing the canonical href for native link behaviors.
• The new context-menu action reuses sessionHref derived from the same helper used by the anchor.

Fixes #460

[roborev-ci]

roborev: Combined Review (cc70d94)

No issues found.

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Panel: ci_default_security | Synthesis: codex | Members: codex_default (codex/default, done, 2m25s), codex_security (codex/security, done, 25s) | Total: 2m50s

[roborev-ci]

roborev: Combined Review (71f8331)

No issues found.

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Panel: ci_default_security | Synthesis: codex | Members: codex_default (codex/default, done, 4m28s), codex_security (codex/security, done, 32s) | Total: 5m0s