Improve stop/edit session date-time and note authoring UX

[Fingolfin7]

Motivation

• Make editing and stopping sessions easier by using native date+time inputs instead of free-form text and providing a lightweight authoring experience for session notes.
• Allow users to dictate notes to avoid repetitive manual typing and add basic markdown helpers (bold, italic, links) to speed up note entry.

Description

• Replaced free-text start/end fields in the session edit form with datetime-local widgets and added server-side input_formats and the existing end-before-start validation in UpdateSessionForm (core/forms.py).
• Added a new StopTimerForm (in core/forms.py) and updated stop_timer to use get_object_or_404(..., user=request.user) and the form for POST handling so the stop-timer page supports editable start/end timestamps and note edits (core/views.py).
• Added a small client-side note editor core/static/core/js/session_note_editor.js that provides bold/italic/link markdown helpers and optional browser Speech Recognition dictation with graceful fallback.
• Wired the editor and controls into the templates for stop and edit flows by updating core/templates/core/stop_timer.html and core/templates/core/update_session.html to include the toolbar and load the new script.

Testing

• Ran python manage.py check to validate Django configuration, but it failed because the environment is missing SECRET_KEY so Django settings could not initialize (no other automated tests were run in this environment).
• Templates and server code were exercised locally in this environment only to ensure imports and syntax are valid (no runtime server start due to missing settings).

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Codex Task