fix(security): cookie-import path validation bypass + hardcoded /tmp

Two fixes:
1. cookie-import relative path bypass (#707): path.isAbsolute() gated the
   entire validation, so relative paths like "sensitive-file.json" bypassed
   the safe-directory check entirely. Now always resolves to absolute path
   with realpathSync for symlink resolution, matching validateOutputPath().

2. Hardcoded /tmp in cookie-import-browser (#708): openDbFromCopy used
   /tmp directly instead of os.tmpdir(), breaking Windows support.

Also adds explicit imports for SAFE_DIRECTORIES and isPathWithin in
write-commands.ts (previously resolved implicitly through bundler).

Closes #852

Co-Authored-By: Toby Morning <urbantech@users.noreply.github.com>

browse/test/commands.test.ts

@@ -1811,7 +1811,8 @@ describe('Path traversal prevention', () => {
       await handleWriteCommand('cookie-import', ['../../etc/shadow'], bm);
       expect(true).toBe(false);
     } catch (err: any) {
-      expect(err.message).toContain('Path traversal');
+      // Traversal blocked by safe-directory check (#707) or explicit .. check
+      expect(err.message).toMatch(/Path must be within|Path traversal/);
     }
   });