feat!: adopt Python 3.14 and 3.15 language features

- Require Python 3.15 for the server and Python 3.14 for the CLI.
- Match the server requirement to quart_common's Python 3.15 features.
- Replace future annotations with native deferred evaluation.
- Defer CLI command dependencies and QR imports on Python 3.15.
- Enable argparse suggestions for mistyped CLI commands.
- Remove the obsolete tomllib import fallback.
- Update my_helpers and quart_common submodule references.
- Lock test dependencies and exclude them from Docker installations.
- Add regression coverage for lazy imports and deferred annotations.
- Document runtime requirements and standalone CLI validation.
- Validate 276 tests on Python 3.15 and 18 CLI tests on Python 3.14.

BREAKING CHANGE: The server now requires Python 3.15+ and the standalone CLI requires Python 3.14+.

cli/nanoshare_client/cli.py

@@ -1,4 +1,13 @@
-from __future__ import annotations
+# Python 3.15 defers command-specific imports; 3.14 imports them normally.
+__lazy_modules__ = [
+  "nanoshare_client.app",
+  "nanoshare_client.auth",
+  "nanoshare_client.client",
+  "nanoshare_client.completion",
+  "nanoshare_client.sync",
+  "nanoshare_client.table",
+  "nanoshare_client.watch",
+]
 
 import argparse
 import json
@@ -120,7 +129,11 @@ def _add_common(parser: argparse.ArgumentParser) -> None:
   parser.add_argument('--node', default=DEFAULT_NODE, help='NanoShare node name.')
 
 def main(argv: list[str] | None = None) -> int:
-  parser = argparse.ArgumentParser(prog='nanoshare', description='NanoShare CLI and folder sync client.')
+  parser = argparse.ArgumentParser(
+    prog='nanoshare',
+    description='NanoShare CLI and folder sync client.',
+    suggest_on_error=True,
+  )
   sub = parser.add_subparsers(dest='command', required=True)
 
   login = sub.add_parser('login', help='Login via browser and write CLI config.')