feat: add DiceBear avatar proxy with style picker on signup and profile pages

Implement avatar generation via local proxy at `/avatar/{style}/{seed}` that fetches from DiceBear 9.x API with in-memory caching and fallback to initial-based SVG. Add avatar style selection dropdown to signup form and profile edit page, storing `avatar_style` in user records. Update comment rendering to support nested threading with parent-child relationships in post view. Add `avatar_url` and `avatar_styles` template globals, register avatar router in main app, and include avatar CSS classes for rounded image display.

tests/conftest.py

@@ -94,10 +94,12 @@ def browser_context(browser):
 def page(browser_context):
     p = browser_context.new_page()
     p.set_default_timeout(10000)
+    p.bring_to_front()
     yield p
     p.close()
 
 def signup_user(page, user):
+    page.bring_to_front()
     page.goto(f"{BASE_URL}/auth/signup")
     page.fill("#username", user["username"])
     page.fill("#email", user["email"])
@@ -108,6 +110,7 @@ def signup_user(page, user):
 
 
 def login_user(page, user):
+    page.bring_to_front()
     page.goto(f"{BASE_URL}/auth/login")
     page.fill("#email", user["email"])
     page.fill("#password", user["password"])
@@ -152,6 +155,7 @@ def bob(browser, seeded_db):
     ctx = browser.new_context(viewport={"width": 1400, "height": 900})
     p = ctx.new_page()
     p.set_default_timeout(10000)
+    p.bring_to_front()
     login_user(p, seeded_db["bob"])
     yield p, seeded_db["bob"]
     p.close()