fix(browser-extension): guard project name against null textContent

[addyCooks]

Fixes #1561

Problem

createProjectSelectionModal declares its onImport callback with a
non-nullable name (utils/ui-components.ts:405-411), but the call site passed
Node.textContent, which is typed string | null. WXT's generated tsconfig
sets strict: true, so check-types failed on the pinned TypeScript 5.8.3:

utils/ui-components.ts(611,13): error TS2345:
Argument of type '{ id: string; name: string | null; containerTag: string; }'
is not assignable to parameter of type '{ id: string; name: string; containerTag: string; }'.

CONTRIBUTING.md instructs contributors to run bun run check-types, so anyone
following the documented workflow hit this.

Fix

Fall back to an empty string. This matches the containerTag line directly
below it and every other .textContent read in the extension all 34 of them
already guard with || "" or ?.; this was the only unguarded site.

selectedOption.text was considered instead, since it is typed non-nullable,
but rejected: per spec it strips and collapses whitespace, which would silently
trim project names.

Verification

Against the exact lockfile toolchain (wxt@0.20.18, typescript@5.8.3):

• Reproduced the error on current main, confirmed it was the only error in
  the package, and confirmed bun run check-types exits 0 after the fix
• Also clean under TypeScript 5.9.3, whose DOM lib does not widen this access
• Runtime behaviour unchanged, verified with 53 assertions over real <option>
  elements: textContent is never null for these nodes, the old and new
  expressions are identical across empty, whitespace-only, padded, emoji,
  newline and 5000-character names, and whitespace is preserved
• The placeholder guard is unaffected: selectedIndex is never -1 and the
  if (selectedOption.value) check short-circuits before name is read
• Biome clean; all 12 existing extension unit tests pass

Runtime behaviour is unchanged this is a type-safety fix.