fix(preview): don't crash the preview panel on a malformed iframeBase URL

[pedrofrxncx]

Follows #6362, which wrapped withDeviceHint/withDecoFBT's new URL() calls in try/catch after a malformed directPreviewUrl/previewUrl crashed the whole preview panel mid-render — matching previewOrigin's existing defensive shape.

It missed four sibling call sites that construct URLs from the same untrusted display.iframeBase (set from the sandbox daemon / production preview server, see preview-display.ts): productionOrigin, the two iframeSrc branches (sandbox and production modes), and productionOpenTabBase. Each still called new URL(...) unguarded as an argument expression — a malformed iframeBase (e.g. an unparseable host) throws before withDeviceHint's own try/catch ever runs, taking the whole panel down the same way #6362 fixed for the other inputs.

Failure scenario: the sandbox daemon or Fast Preview server reports an iframeBase that isn't a valid absolute URL (malformed host, stray bracket in an IPv6 literal, etc.) — the preview panel throws during render and goes blank instead of degrading gracefully.

Fix: added resolvePreviewUrl(), the same try/catch-and-fall-back-to-null shape as the existing previewOrigin helper, and used it (or previewOrigin itself, for the origin-only case) at all four call sites. Added a regression test mirroring the existing withDeviceHint/withDecoFBT malformed-URL tests.

To verify: bun test apps/web/src/components/sandbox/preview/preview.test.ts

Checked locally: bun run fmt, cd apps/web && bunx tsc --noEmit, bun test apps/web/src/components/sandbox/preview/preview.test.ts (7 pass), bunx oxlint on both touched files (0 warnings/errors). Full CI validates the rest.

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Summary by cubic

Stops the preview panel from crashing when display.iframeBase is malformed. Previously, new URL(...) at several sites threw during render; now URL resolution is guarded and falls back instead of throwing.

• Add resolvePreviewUrl(path, base): try/catch, returns href or null, same defensive shape as previewOrigin.
• Use guarded resolution at four sites: productionOrigin, sandbox iframeSrc, production iframeSrc, and productionOpenTabBase.
• Behavior with malformed base: panel stays up; iframe may fail to load; “Open in new tab” may be disabled when the URL is null. Valid bases behave unchanged.
• Add regression tests for resolvePreviewUrl. Verify with: bun test apps/web/src/components/sandbox/preview/preview.test.ts.

^{Written for commit 093e6ac. Summary will update on new commits.}