Monetization: Polar chosen as billing provider

- Updated ai.md billing decision to reference Polar (polar.sh)
- Updated code comments (PremiumUrl, IsPremium) to reference Polar
- MONETIZATION.md updated locally (gitignored — confidential)

207 tests, 0 warnings.

ai.md

@@ -609,9 +609,9 @@ annual subscription** (settled 2026-08-14 — replaces the earlier "one-time unl
   keep the promise simple).
 - **Billing:** NOT locked to itch.io (creator's call 2026-08-14). Hunted fact — **itch.io has no native
   subscription billing** (no annual/recurring product billing; only pay-what-you-want, pre-orders,
-  early-access, keys, and a Patreon integration). Candidates for annual subs + license-key verification:
-  **Gumroad** (its native affiliate program is a tiebreaker if the affiliate angle in TASK 10 matters)
-  and **Lemon Squeezy**. Decision deferred to TASK 10.
+  early-access, keys, and a Patreon integration). **Chosen platform: Polar (polar.sh)** — open-source
+  MoR (Apache 2.0), handles payments, subscriptions, license keys, and global tax compliance.
+  Startup Program gives Scale plan free for 12 months. Details in `MONETIZATION.md`.
 - **Support (creator's model):** in-app bug-reporting mechanism → issues into git; most queries are
   how-tos / feature requests / manual-skimmers. Maintenance cadence = "when I get around to it" with
   emergency patches; not a 24/7 service promise.