[Snap] Show actionable warning when password-manager-service is not connected instead of falling back to master password silently

Describe the bug / feature request

When pgAdmin 4 is installed via snap and the password-manager-service
interface is not connected, pgAdmin silently falls back to requiring a
master password with no explanation why.

Since pgAdmin 7.2+, the OS keyring is the default and preferred storage
for saved passwords in desktop mode. However, snap's strict confinement
prevents keyring access unless the user explicitly runs:

snap connect pgadmin4:password-manager-service

This is not documented anywhere in the app, and most users will never
find it. The only hint is in the pgadmin4-snap GitHub README, which most
users never read.

Expected behavior

When pgAdmin detects that the OS keyring is unavailable (e.g.
NoKeyringError), it should:

1. Show a clear, user-friendly message explaining that the system keyring
   is not accessible
2. On Linux snap installs specifically, suggest the fix:
   snap connect pgadmin4:password-manager-service
3. Offer master password as a fallback option with explanation

Current behavior

pgAdmin silently falls back to prompting for a master password with no
explanation, leading users to spend significant time trying to disable
the master password via config files (which is impossible to do via
config_local.py on snap due to read-only filesystem and snap Python path
isolation).

Steps to reproduce

1. Install pgAdmin via snap: sudo snap install pgadmin4
2. Open pgAdmin
3. Observe master password prompt with no explanation

Fix

The fix in master_password.py where NoKeyringError is caught could
be extended to surface a user-facing message. Additionally, requesting
auto-connect approval from the Snap Store team for the
password-manager-service interface would solve this entirely.

Environment

• OS: Ubuntu (snap install)
• pgAdmin version: 9.14
• Package type: Snap

Start in master_password.py at the NoKeyringError catch and follow how the master-password prompt is surfaced; reproduce the documented snap installation with the password-manager-service interface disconnected. Done means the unavailable keyring is explained, the snap command is suggested where applicable, and the master-password fallback is clearly identified.