docs(browser): add permissions guide and improve documentation

- Create comprehensive PERMISSIONS.md explaining all permissions
- Document empty host_permissions array and privacy benefits
- Add session cookie security best practices to BrowserExtension.md
- Clarify future use case for content script message handler
- Improve error message clarity for JSON parsing failures

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

browser-extension/popup/popup.js

@@ -121,7 +121,8 @@ sendFileBtn.addEventListener('click', async () => {
             try {
                 const result = await response.json();
                 errorMessage = result.detail || errorMessage;
-            } catch (e) {
+            } catch (jsonError) {
+                // Server returned non-JSON error response
                 errorMessage = `HTTP ${response.status}: ${response.statusText}`;
             }
             showStatus(`Error: ${errorMessage}`, 'error');