Add --visualize option to admin assignments

[saimeda32]

The CSV that admin assignments prints is complete, but it gets hard to review once an org has more than a few accounts. Ours is around 230 rows, and answering a question like "what can this group reach" means grepping.

This adds a --visualize option that writes the same results as an interactive HTML page, next to the normal CSV output:

aws-sso-util admin assignments --visualize

What the page does:

• three columns (principals, permission sets, accounts) with a line per assignment, line weight reflects assignment count
• hover to preview, click to pin a node and trace its full paths; the side panel lists them and every name in it is clickable, so you can jump from a group to an account to another group
• search with a result picker, or trace all matches together
• plain table view for when you just want rows, dark mode via prefers-color-scheme

Implementation notes:

• the template ships as package data next to the module; rendering is a placeholder replacement in visualize.py, no new runtime dependencies
• the JSON is escaped when injected (< and the JS line separators), so a hostile principal or account name cannot break out of the script element
• accounts are keyed by account id rather than name, since account names are not unique
• all existing filters (--group, --permission-set, --ou, and so on) apply to the graph too; the CSV output is unchanged; the filename is optional and defaults to access-graph.html

Testing:

• pytest suite for the renderer under cli/tests (9 tests); pytest added as a dev dependency
• ran it against a real Identity Center instance with about 230 assignments across 13 accounts, and exercised hover, pin, search, trace all, table view, dark mode, and keyboard input in a browser
• the screenshot above uses made up example data

Docs: new section in docs/lookup.md plus a pointer in the README.

Happy to adjust naming, restructure the template, or drop the screenshot from the repo if you prefer to keep images out of it.