Update to Leo's product header

[aguscruiz]

Description

Changes:

• Font size changes to 19px with 19px line-height (uses a Nala style that needs to be uploaded called Sidebar product title)
• The header should have a 60px minimum height. But give the top-bottom paddings a 16px value in case it needs to grow a bit more.
• Product title is always lowercase
• Product name changes to "leo ai"
• Added a "close" icon on the right to close the panel (Desktop and Android)

Design
https://www.figma.com/file/MNwi0uwXNhkAbPiJ86G4vm/%E2%9C%A8-Leo?type=design&node-id=1506%3A7892&mode=design&t=i7noPdyJ4idIyTyS-1

[stephendonner]

Verification PASSED using

Brave | 1.67.31 Chromium: 124.0.6367.60 (Official Build) nightly (64-bit)
-- | --
Revision | 5d142aa04ae5b4f6447932c461da4cda17db69d9
OS | Windows 10 Version 22H2 (Build 19045.4291)

Confirmed:

• letter-casing and string change to leo ai from Leo
• new Close widget with correct tooltip of Close

non-Premium - PASSED

Steps:

1. installed 1.67.31
2. launched Brave
3. loaded vox.com
4. opened the sidebar
5. clicked on Summarize
6. clicked on I understand at the opt-in screen

Dark	Light

Premium - PASSED

(continued from non-Premium steps)

Steps:

7. loaded account.bravesoftware.com
8. purchased Leo Premium and relaunched with --env-leo=staging --env-ai-chat.bsg=dev --env-ai-chat-premium.bsg=dev
9. confirmed Leo Premium credentials/badging

Dark	Light

[Uni-verse]

Verified on on Samsung Galaxy S21 using version:

Brave	1.67.92 Chromium: 125.0.6422.76 (Official Build) beta (64-bit) 
Revision	d751d2edf40b466560aaeecc4c4919041f41c20d
OS	Android 13; Build/TP1A.220624.014; 33; REL

• Ensured that product title is always lower case.
• Ensured badging is correct for free and premium users.
• Ensured tapping on the X closes Leo chat and tapping on eraser icon will start new conversation.
• Ensured cog icon will open the Leo main menu.
• Ensured there are no UI or design issues when upgrading from previous versions with Leo already enabled.

Example	Example	Example