Implement PKCE support

[Doridian]

Based upon #162, otherwise merge conflicts are annoying

This is the frontend part of PKCE support.

Fixes donetick/donetick#739

[tatezach]

This PR fixes a real, reproducible bug that isn't mentioned in its title, so I want to document it here — it may be worth pulling the generateRandomString change out as a standalone fix if PKCE review takes longer.

The current state generation is too short for spec-compliant providers. On main:

const randomState = Math.random().toString(32).substring(5)

Math.random().toString(32) returns 0. followed by a variable number of base-32 digits; stripping five characters leaves roughly seven to nine. Authelia enforces minimum_parameter_entropy: 8 by default and rejects anything shorter:

error=invalid_state
error_description=The state is missing or does not have enough characters and is
therefore considered too weak. Request parameter 'state' must be at least be 8
characters long to ensure sufficient entropy.
&state=nso2l4v

That state has seven characters. Because the length varies per attempt, login succeeds sometimes and fails other times, which makes it look like a flaky provider or reverse proxy rather than a client bug. The only workaround today is minimum_parameter_entropy: -1, which Authelia explicitly discourages in its own documentation.

This PR's generateRandomString(16) fixes it properly: 22 base64url characters, and crypto.getRandomValues instead of Math.random(). The latter matters independently of length — state is the CSRF protection for the authorization code flow, and Math.random() is not cryptographically secure.

For the record, this was reported in #17 back in March 2025 with the identical Authelia error and a six-character state. That issue was closed as completed, but the code on main was never changed, so the behaviour has persisted since then.

One review note: Uint8Array.prototype.toBase64() is very recent and not available in older WebViews. Since the same code path runs inside Capacitor on Android and iOS, it may be worth using a manual hex or base64url conversion instead, so the auth flow doesn't break on older devices:

const randomState = Array.from(data, b => b.toString(16).padStart(2, '0')).join('')

Happy to test a build against Authelia v4.39 on web, Android and iOS.